Disable all Firefox addons, see if that helps. Make sure your looking at HDD read/writes and not memory read/writes. If you've added a lot of files or a new hard drive, or a new email account with thousands of emails lately SearchIndexer may be re-indexing your hard drive. If you haven't I would suggest resetting Search Indexer's database.
Start, Run, "control /name Microsoft.IndexingOptions", Advanced, Rebuild. Note that you will see lots of HDD activity after rebuilding this, which is normal. It may take hours for it to go away, and Indexing performs faster if the machine is left idle.