I have used Firefox as my sole web browser for a long time now, and the only problem I have previously encountered is where I cannot upload a photo when selling an item on eBay.
However, I have now updated to v3.6 and have the following additional problems:
1. eBay - occasionally, when I click on an action button, it comes up with an error page stating that the system is unavailable.
2. Royal Mail site - when using 'Postcode Finder' the 'security code' box (eg 'type the digits as shown') does not work correctly. No matter how many times I input the correct characters it says I have to retry.
Both the above problems have been solved by using Google Chrome.
I would get used to the problems you are experiencing on Firefox and Opera to. The developers of both these browsers seem to think Google Chrome is the one to follow. As a result you will see big changes in both browsers and a greatly sped up release cycle. Both of these things will result in buggy browsers and drive people into using Google Chrome. Both Firefox and Opera are shooting them selves in the head by going down this path. I hope I`m wrong but recent events seem to be confirming my concerns.
Try using Minefield (I know, I know, it has nightly builds that are 'bound to have crashes and other headaches), but I haven't had anything like that, and it's been quite stable for me. I think they might have fixed some of those bugs in the latest Minefield versions, and the new default theme of it is awesome.
Firefox (all versions I've ever used) have some funky handling of the text window on forums that use Invision Power Board like this forum uses, in that if you want to edit some text and highlight it or do something else Firefox may on some rare occasions freeze for several seconds to several minutes before it can be used again. I've had that issue with and without using any add-ons, and with two completely different Windows installations.