There are ways of doing this, but apparently the current official policy is that you either revert to firefox 3.6 or stay on firefox 5. ( see installing a previous version of firefox )

If you do stay on Firefox 5 and the lost abilities relate to add-ons then try to use this add-on https://addons.mozilla.org/en-US/firefox/addon/add-on-compatibility-reporter/

If you continue to have problems and need to re-install firefox 4 please detail the problems, and the problem add-ons, I or someone else will I am sure be able to offer additional information.

5.0 is not compatible with my full-time employer's website yet. I am having difficulty accessing sites/pages that I need to perform my job. I need to revert and I prefer 4.0 to 3.6.

It may be useful to know what add-ons or websites are a problem, if they are publically available they should hopefully be compatible with Firefox 5. If you do specifically need Firefox 4, the direct link was removed last time I included it in a post but have a look closely at this information: http://kb.mozillazine.org/Go_back_to_an_old_version_of_Firefox