... What version or FF are you using?
A few tips.
1. AVOID BETA VERSIONS AT ALL COST!---Betas are used for de-bugging and what better bug finders than the general public. If you are still using 1.4.5b, then upgrade to 1.5. The Mozilla staff save money on a de-bug team by releasing it to the people of the world to report bugs to them for free. Therefore, they are fulll of bugs and Mozilla wants you to find them and they will fix them in the newest version.
2. Don't over-open tabs. Everyone does it. I have three open at the time being. It overloads the bandwidth being input into that one window and it often chashes then. Do as I do... When you reach a fourth tab, open a new Window instead and work from there. Don't overuse the windows either. For some computers, they have just a little RAM left when the Photoshops and Flash Games are open and opening more and overloading your RAM could lead to the Blue Screen Of Death for windows. Instead, use Linux Red Hat if you can for a default OS. It doesn't use up as much RAM and is much more compatible with Mozilla than IE.
3. Don't close your screen. Firefox is loaded with protection that you may not know about. It will automatically delete your cookies that you downloaded onto FF when you close all FF windows. That would solve your login problem. The cookie that contains that password and username is deleted for saftey reasons (unless you save it when it prompts you). That is to prevent someone coming into your house, looking at history (it will also delete that if you set it) and visiting a page that you were logged into and still be logged in... Especially in the case of a bank account, as where you could lose all your money.