You can get older versions from filehippo.com - all the way back to Firefox 1.0 if you're so inclined.

Personally, I'm not downloading Firefox version updates any longer. They don't address major issues such as the memory leak problem, they don't add any significant functionality, they don't join the 22nd century with a 64-bit release, they don't give the user the option to have the browser function as an SDI application - all they do is break things and cause annoying problems.

I stopped downloading updates once I had finally fixed all the problems caused by updating to Firefox 4, and I will stick with that until I find something better.

Frankly, I rate the chances of Mozilla actually ever giving us that "something better" to be precisely zero.

^ Ah, a time traveler, who is visiting those of us who live in the 21st century from the future!

Firefox 5 was a security up date for Firefox 4, and is no longer supported which is why you are being offered Firefox 5 instead.

The Google Toolbar "will not be supported on Firefox 5 and future versions."
http://googletoolbarhelp.blogspot.com/2011/07/update-on-google-toolbar-for-firefox.html

Time to look into alternatives to be able to access the Google features which the GTB supported, or use the IE version of the GTB.
http://kb.mozillazine.org/Using_Google_Toolbar_features_without_toolbars