If you are in the habit of checking the "tools", check for updates" section in Firefox, DON'T install Firefox 1.01 over Firefox 1.0. UNINSTALL 1.0 first, saving your profile (go check mozilla forums to see exactly how).
I didn't. The browser is now inoperable and it also won't uninstall either version.
Links later when I figure out exactly how to fix this- as I have to dash out of here right now.
Revised, added, updated 3/3/05: I finally have Firefox 1.01 working, with my bookmarks and most of the various tweaks that make it most useful to me- intact. The problems were created by me, by not reading well or fully or paying attention as I should have. But I was not the only one. "Update" and "Critical Update" mean something different to most average folks.
The process to fix this may well be individual (if you installed on top of 1.0 as I did), as several things that I tried did not work automatically. For instance using XP's "restore" going back to before I installed the "update", did not fix the problems (I don't understand why either, but that is another story) , and uninstalling was not a one click operation. Not everything uninstalls and it requires a good search of your system to remove all the various places that have little crumbs left, before reinstalling (save the profile information!!). Oh, and don't forget to check your firewall after you uninstall and reinstall; you may need to re-allow Firefox access to the internet.
They may have an update now that really will install over the 1.0 version, but just in case, my warning about reading first, still stands. And if you are/were inattentive and slower about technical things as I am and was- check out the Mozilla Firefox Support Forum before you do anything.
added 05/05/05: If you are looking for more current information on Firefox, you will find a summary and link of every post I have made to date about the various versions of Firefox, any installation problems and my solutions; and within some of those posts are links to specific Firefox support forum threads. Firefox post list