It is possible. But what are you trying to achieve in the first place?

want to see if there are fixes in the current TB version for client cert processing and at same time not lose my working old version.

You really don't get a choice Joe. With the release of Thunderbird three there were changes to the actual profile folder structure. Since then there have been additional changes to where information (mostly around passwords and certificates). So the profile is not compatible with later versions. As a matter of fact your update will most likely fail unless you install V3 or 5 in the interim to get the relevant conversions to the profile.

You can make a backup copy of your profile to safe guard yourself. but at some point your going to have to update.