You neither described the problem you actually had nor the version you had. There may be other solutions such as described in the following answer which also tells you how to go back to to any version

https://support.mozilla.com/questions/840344

You can make Firefox 4.0.1 and Firefox 5.0 look like Firefox 3.6.17, see numbered items 1-10 in the following topic Fix Firefox 4.0 toolbar user interface, problems (Make Firefox 4.0 look like 3.6)

I just want to go back to 3.6.17 that worked great for me. I lost TABS when it went to 3.4.18, so I downloaded 5.o. No the mouse is makes jerky movements no Back drop down button and who knows what else I'll find that I don't like.