Yes. You can instal multiple versions in different folders and use a separate profile for each one. The downside is that they won't share bookmarks and such. So if you are only going to use 3.6 for time-sheets, go ahead and update your main firefox to the version you plan on using, and instal 3.6 in a different folder (I just tack the version number onto the end of the folder name for mine).

Then you are going to want to create a new profile for the 2nd version. There is an article here that explains how to do that:

http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox#Using_new_profiles_in_secondary_Firefox_installations

More info on profiles can be found here: https://support.mozilla.com/en-US/kb/Managing-profiles

Firefox 3.6 can be downloaded here: http://www.mozilla.com/en-US/firefox/all-older.html

You could instead install Firefox Portable on your hard drive or a USB drive and not need to set up multiple profiles. Portable keeps all of its Extensions, Plugins, etc., within the Portable app Program Files folder.

General info: http://portableapps.com/support/firefox_portable
Link to download: http://portableapps.com/apps/internet/firefox_portable

• Note for older versions, click "Other versions", under the big "Download 4.0.1" button.
• Note that the link for version 3.6.17 will go to Firefox version 3.5.x; just scroll up a bit.

Might be easier to use Firefox Portable 3.6.X, you won't have to mess around with "Profiles". Portable installs to its' own folderset and won't affect another Firefox installation.

http://portableapps.com/apps/internet/firefox_portable/localization#legacy36