Hi Barry,
I deliberately run multiple versions of Firefox, but yes standard installations tend to be of one version only, and are often updates that result in only the one installation. Presumably at some stage you have done a custom installation.

Take care in removing or uninstalling Firefox, it will offer to remove personal settings, or however it is worded, do NOT use that option, because it is likely it will remove all bookmarks and settings for all versions of Firefox.

Another thought is do these two versions share the same profile so do you see exactly the same bookmarks and history in each version. (They probably do unless you or someone else has deliberately set up a separate profile, but worth checking, before you remove the older version)

So summing up yes it is safe to remove the older version, simply use the uninstall option from XPs control panel, but remember not to delete your settings.

If you really see two installation folders on your computer then you can remove (uninstall) the older 3.5.x version.

If you refer to an entry in Add & Remove Programs then it is possible that you only see a leftover (orphaned) Firefox 3.5 entry and that the 3.5 version has been updated and added a new entry for Firefox 5, but didn't remove the entry of the previous Firefox 3.5 entry in Add & Remove Programs.
In that case you may uninstall the current Firefox 5 version instead if you remove Firefox 3.5.

@ John99 - Interesting - my problem is, I want to find out how to have multiple versions of Ffox on my systems (mostly XP, also Mac OS) without ending up in a tangle. (I support a number of lightweight websites with mostly very non-tech users, hence need to test with old versions!) I have exhausted my imagination in constructing search queries, and your post is the only mention I have found! I've also looked at /developer.mozilla.org/ without success. Can you help please!

- apologies - can't delete this accidental duplicate -

Do a custom install and install each version in its own program folder to use multiple Firefox versions.

• https://support.mozilla.com/kb/Custom+installation+of+Firefox+on+Windows

Create a new profile exclusively for each Firefox version.
Create a desktop shortcut with -P "profile" appended to the target to launch each Firefox version with its own profile.

See:

• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox
• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile

• http://kb.mozillazine.org/Testing_pre-release_versions

Many many thanks Cor-el for the swift response - I will follow up as soon as I can. Do the items address the Mac OS locale - or is it fairly obvious how the same principles apply?