Yes, that's correct, the version of Firefox you have installed is too old.

Please do the following.

1. Go to Download Firefox 4.0.1 and download the installation file to your desktop.
   
   
2. Then go to Add/Remove Programs, scroll down to "Mozilla Firefox" and remove it, choosing to keep your bookmarks, customizations etc., (don't checkmark the box).
   
   
3. Then reboot and delete the folder called "Mozilla Firefox" at this location: C:\Program Files\Mozilla Firefox
   
   
4. Finally run the installation file (that means double click it) you downloaded to the desktop earlier.
   

Your bookmarks, customizations etc., are maintained in a different location and will become available to you again once you complete the installation.

See this:
https://support.mozilla.com/en-US/kb/websites+or+add-ons+incorrectly+report+incompatible+browser

Ed, it's not a good idea to keep the version jennifer is using because it's a security risk.

@ jennifer, an SSL Provider (SSL is what banks use to encrypt sites) called Comodo experienced a security breach recently which resulted in a number of fraudulent certificates being issued. These certificates will allow criminals to create a fake banking site which looks exactly like your own and the version of Firefox you have installed at the moment will not warn you that it's fraudulent.

Comodo's blog explains the risk here: http://blogs.comodo.com/category/it-security/data-security/

These certificates were blacklisted in Firefox 3.6.17 and beyond. The version I linked to is the current one.

Looks to me as well as a corrupted user agent, so check out the Knowledge Base article in Ed's post and reset the pref general.useragent.override to the default (empty string) on the about:config page.

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.
You can use the Filter bar at to top of the about:config page to locate a pref more easily.