I'm a little confused about what version you had before. If Firefox 5 is not working well, you may want to roll back to Firefox 3.6. The reason is, Firefox 4 has security issues that are only fixed in Firefox 5.

Here's the process to roll back:

First, I recommend backing up your Firefox settings in case something goes wrong. See Backing up your information. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, download and save Firefox 3.6 to your desktop for future installation. http://www.mozilla.com/firefox/all-older

Close Firefox 5.

You could install Firefox 3.6 over it (many have reported success) or you could uninstall Firefox first. If you uninstall, do not remove your personal data and settings, just the program.

Unless you have installed an incompatible add-on, Firefox 3.6 should pick up where you left off. If there are serious issues, please post back with details.

Note: I haven't actually tried this myself!

While you can get Firefox 4 here, it's not recommended because it doesn't get updates (rendering it as an unsafe version to use). You can, however, follow the instructions of the other poster to install FF 3.6, which is still supported.