well...the OP jinxed me!!!
just started acting up...and majorly at that! found a lot of threads (had to use my MSN browser

), and there are MANY people having this issue.
my tired brain then remembered that this has happened to me back with FF 3.0.x ....there IS a fix, but:
1) it should not be necessary, the Mozilla gang should have a tool to do some sort of cleanup of FFox's file system.
2) while I can show you how? it WILL take a bit of work (though not long/hard)
-> keep in mind, that you CAN just uninstall and delete FFox...but then you will lose your passwords and bookmarks...if you what to save all of that? read on...
- - - - the FireFix - - - -works for any version BTWay
seems that FFox is NOT geared to the abuse that some of us 'surfers' put to it, and over time the files in your profile (even IF you do not have one, there is the 'default' profile) and it gets all crapped up, corrupted, and fragged...
so this is what you do - you will need to make a fresh profile and move-> into a fresh clean house! (I
just did this and life is back to normal...it works)
1- depending on how badly FFox is behaving? you may find that you can NOT click on links or enter more than 1 URL before things lock up...so do NOT enter any URLs after it boots...
2- start up- go to 'bookmarks->organize->import&backup->backup ... just drop this onto your desktop.
3- open up 'tools-add-on' and make a note off what you have installed...sorry this is NOT currently 'saveable'
4- copy this URL, (in case you have to try multiple times!) , and drop it in notepad...turn 'wordwrap' "off"
https://addons.mozilla.org/en-US/firefox/downloads/latest/2848/addon-2848-latest.xpi?src=addondetail
paste it into your (currently) broken FFox...and give it a go...you may have to try a few exit and restarts (if you get the old session restore? choose 'new') this will download the ID/password exporter...
4a- again FFox may be clonky, and you may have to try several times to get the tool to 'go' (mine kept ignoring the 'I accept' button <-this is because your passwords will be pulled out into a speadsheet...you can always delete it later or lock it into a password protected 'zip' (though do NOT forget the password for that!

)
sooner or later the file will be generated, again drop it on your desktop for easy finding...
5- almost done! in XP go to the 'start' button and hit 'run' or hit the windows key+R (Vista may need this, as the run menu is often 'off')
enter firefox -p
this will run FFox in "choose a profile mode" - first, I recommend that you leave the ‘Don't ask at startup' box
unchecked - this way you can also now switch...create a new profile...I used Dec09 so I can remember when I created the new profile.
you are now running 'clean'...so go to the URL above again and install the password add-on...exit, restart, choose your new profile (doh?)...go to add-ons, and import that .xml file on your desktop
...now go to 'bookmarks->organize->import&backup->restore-choose file' & load in the .json file cleverly named "bookmarks-2009-xx-xx.json"
- other than reloading themes and add-ons (extensions only, oddly the plugins are universal for all profiles?) you should now be gold for 12-18 months...since you are 'working' on FFox why not tweak it as well? both if these programs seem to increase FFox's speed a bit to a lot, depending on the site. I just re-ran them as well
http://www.softpedia.com/progDownload/FireTune-Download-20945.html
http://www.softpedia.com/progDownload/Mozilla-Optimizer-Download-9693.html
hope that gets some of you blasting about your URLs again...still this should NOT be needed, and at the end of the day? while it works, it is not a true fix, as over time you will end up with several broken profiles (which you can and should delete after a while)
R-