Firefox about:config: Full URLs, Safer Ctrl+W, Readable Links

Firefox's about:config Editor Rewards Precision, Not Exploration​

You open the Configuration Editor by typing about:config into Firefox's address bar. It lists advanced preferences that Firefox reads from your profile files and from the application's defaults. Many of them have no switch in the regular Settings panels. Before you get in, Firefox shows a warning page and an "Accept the Risk and Continue" button. Mozilla's support documentation says changing these values can seriously affect browser stability, security and performance. It describes the editor as a tool for experienced users, says Mozilla does not officially support changes made there, and warns that Firefox updates can break advanced customizations.

The MakeUseOf piece handles that warning sensibly. Onyimadu says he left networking, DNS, fingerprinting and performance preferences alone whenever he couldn't confidently explain what they would do. He kept these three because their effects are easy to see and easy to reverse. That is his own rule of thumb, not a finding about any particular setting he skipped. It is still a good way to approach a page with hundreds of cryptically named entries.

The mechanics are simple and don't vary between these three. They are all Boolean preferences, meaning each is either true or false. According to Mozilla, you flip a Boolean by double-clicking its row or clicking its Toggle button. Modified preferences show in bold, and a Reset button puts a changed value back to its default. Preference names are case-sensitive, so type or paste them exactly.

browser.urlbar.trimURLs Trades a Tidy Address Bar for an Exact One​

Firefox tidies URLs in the address bar by default. The browser.urlbar.trimURLs preference controls this, and its default is true. Set it to false and Firefox shows more of each address, including parts it would otherwise hide.

This only changes how the address is displayed. It doesn't change which site you load, and seeing a fuller URL doesn't make a connection any more secure. Onyimadu notes the effect is most obvious on plain HTTP sites, where trimming changes how parts of the address appear.

His best example matters to anyone who writes documentation or makes tutorials. With trimming on, a router admin page at [url]http://192.168.1.1/[/url] shows up as just 192.168.1.1. If a reader copies that shortened address from a screenshot or video and types it in, Firefox may treat the input as a search query rather than the local device address. For anyone writing guides about routers, NAS boxes, self-hosted services or other LAN devices reached over http://, turning trimming off means screenshots show the address the reader should actually type.

It also helps with everyday checks. Onyimadu likes seeing the fuller address when he's following a redirect, comparing two similar links, or simply wants to see exactly what Firefox has loaded. If the busier address bar gets annoying, set the preference back to true or click Reset.

browser.tabs.closeWindowWithLastTab Turns Ctrl+W Into a Safe Reflex​

This one makes the most obvious difference. Out of the box, closing the last tab in a window closes the whole window. If it's your only window, the result is the one a user described on Mozilla's support forum: "When I close the last tab I am using, the Firefox browser disappears."

The preference has been around for a long time. A 2018 support-forum answer quoted the old MozillaZine reference describing browser.tabs.closeWindowWithLastTab Boolean (Firefox 3.5 and above) This preference controls whether closing the last tab, closes the window. Mozilla forum volunteers have suggested setting it to false for more than a decade, from 2012 answers through to Firefox 77-era threads.

With the preference set to false, a window without tabs doesn't close. Instead, when you close the last tab, Firefox will display either your home page or a new tab page. One volunteer tested in Firefox 77 on Windows 7 by setting this preference to false. When I tried to close the last tab, Firefox opened the New Tab page. To close Firefox, I had to use the X at the top of the window or go to the Firefox 3-bar menu and click Exit. A 2026 SysTutorials guide says the change takes effect immediately, so you don't need to restart the browser.

Onyimadu's reasons will sound familiar to keyboard-heavy users. He presses Ctrl+W by reflex and doesn't always check how many tabs are left. Hitting it on a lone tab can wipe out window-specific state, such as a developer tools panel or an open about:config session. He says reopening closed tabs doesn't always bring that context back. He also finds the setting useful on multi-monitor setups and with tiling window managers, where losing a window means rebuilding it and putting it back in place.

The trade-off is plain: if you close windows on purpose by closing their last tab, this setting gets in your way. You'll need the window's close button or the menu's Exit command instead. It also interacts with session restore. A Mozilla forum volunteer explained that once the last tab becomes a home or new-tab page, if you then close the window and it's the last window, that is what will get restored. If you rely on "Restore previous session", expect a blank page rather than no window at all.

browser.urlbar.decodeURLsOnCopy Makes Pasted Links Readable, With a Compatibility Catch​

The third preference controls what ends up on your clipboard. Many URLs contain percent-encoded characters, where a character outside plain ASCII is written as a % followed by hex codes. Take a Wikipedia article on the Café de Flore. The address bar shows the accented é, but by default the copied link pastes into Slack, Word or email as [Café de Flore - Wikipedia](https://en.wikipedia.org/wiki/Caf%C3%A9_de_Flore). The link works fine; it's just harder to read.

Firefox's source confirms that the setting still exists and what it does. In Mozilla's current address-bar preference definitions, decodeURLsOnCopy sits under the browser.urlbar. branch with a default of false. The comment next to it says it controls whether copying the entire URL from the location bar puts a human-readable, percent-decoded URL on the clipboard. Set it to true and copying the Café de Flore address gives you the accented é.

Mozilla also has a reason for the default. In a Bugzilla report filed against Firefox 135 (bug 1941325), a user complained that Firefox showed a space in the address bar but encoded it as %20 when copying. Mozilla address-bar engineer Marco Bonardo closed it as WONTFIX. He explained that Firefox decodes the displayed URL for readability but encodes the copied one because some third-party apps don't cope well with decoded URIs, and he pointed to browser.urlbar.decodeURLsOnCopy as the way to change that.

So this is a formatting choice. It doesn't improve privacy or security. Onyimadu keeps it on because most of the places he pastes links handle the decoded form fine, but he stops short of calling it universally better, since some services expect or handle the encoded form differently. If you often paste links into ticketing systems, scripts, config files or older apps, the default is the safer choice. If you mainly share links in chat or documents where people need to read them, turning it on is a reasonable change.

How to Change and Undo the Three Firefox Preferences​

The steps are the same for all three, and they apply to desktop Firefox on Windows. Keep a note of each original value so you know where you started.

1. Open a new tab, type about:config in the address bar, and press Enter.
2. Click "Accept the Risk and Continue" on the warning page.
3. Paste the exact preference name into the search box: browser.urlbar.trimURLs, browser.tabs.closeWindowWithLastTab, or browser.urlbar.decodeURLsOnCopy.
4. Double-click the row, or click its Toggle button, to switch the value. For the first two, the change is from true to false. For decodeURLsOnCopy, it's from false to true.
5. Check that the row now shows in bold, which means it differs from Firefox's default.

To check each change works: with trimming off, an HTTP address should show its full form in the address bar. With the tab setting changed, pressing Ctrl+W on a window's only tab should leave you on a new tab or home page. With decoding on, copying a URL with accented characters from the address bar should paste those characters rather than % codes. If you don't like the result, search for the preference again and click Reset.

What this means for you​

Try whichever of these three fixes a problem you actually have, and don't treat them as a general tune-up. Each changes one behavior you can see: what the address bar shows, whether a window survives losing its last tab, or how a copied link is formatted. None of them affects privacy, performance or security, so there's no reason to turn on all three just because you're in the editor.

• If you write guides or record tutorials that involve local http:// devices, set browser.urlbar.trimURLs to false so screenshots show the address readers need to type.
• If you're a habitual Ctrl+W user or run multiple monitors, set browser.tabs.closeWindowWithLastTab to false, and use the window's X button or the menu's Exit command to close Firefox.
• If you use session restore, expect a window that ends on its last tab to come back as a new tab or home page once this setting is on.
• If you mostly paste links into chat and documents, set browser.urlbar.decodeURLsOnCopy to true. Leave it at false if you paste links into scripts, tickets or older apps that may not handle decoded URLs.
• If a change bothers you, use Reset in about:config, and don't touch networking, DNS or fingerprinting preferences unless you can explain what they do.

In most cases, the useful parts of about:config are small, specific toggles like these three. Mozilla says it doesn't officially support editor changes and that updates can affect them, so after a major Firefox release it's worth checking the bold rows are still set the way you left them. As long as they are, you get a more exact address bar, safer Ctrl+W and cleaner pasted links, and none of it is harder to undo than to set.