I avoid that problem by using PortableFirefox and installing it to a separate NTFS partition of my HDD.
I also use this separate partition for PortableThunderbird and PortableOpenOffice as well as a place to install MS Works and other large programs.
The Portable installs are completely customized with all plugins and add-ons and completely self contained.
These Portable Apps were designed by a gent named John Haller to be used from a USB thumb drive and as such don't "install" to the host system (no registry entries etc.).
Installing to your harddrive creates a self contained folder with all the components to run the apps.
I just make shortcuts to the executables on my desktop.
One advantage of this is that flash and other plugins are already installed and active.
Another advantage is you can move the folders to another location, even another Windows computer simply by copying the folders to the new location and creating shortcuts to the executables.
I've been doing this for years and my present Firefox 3 install is completely up to date and I have a copy of it tucked away in case the original gets damaged.
My Tbird is completely setup with all my various pop3 email accounts (2 gmail, my ISP account and my Hotmail)
along with my current addressbook. I also have a backup copy.
If you guys are interested here is a link to Hallers site.
Applications | PortableApps.com - Portable software for USB drives
As you can see Haller etal have been busy.
Great stuff.
BTW, when an update to Firefox, Tbird and others is released you can use the update feature in the Portable versions just like in the static installed versions, including add-ons and plugins.