What can I do to preserve tabs when restarting Firefox?

"Go to Edit > Preferences > Main > When FireFox starts and select Show my windows and tabs from last time. Once. After that, you can close FireFox, and it'll be remember everything you have open to show you next time you start it

[edit] For Firefox 5 on Windows, the option is located at:
Tools > Options > General > When Firefox starts: Select ""Show my windows and tabs from last time"""

"You seem to have two disjoint parts in your question.

enable/disable plugins which leads to a restart requirement with firefox

intent of adding new plugins which would also require a restart of firefox

If you have already done point 1 and want to now do point 2 without causing effect of actions in point 1, you will need to undo the enables and disables you did there.

All this can be done without a restart.
If the session manager is around (as others point out), it will restore your tabs after the restart."

You can try Firefox' session restore page (about:sessionrestore), see if it helps.

"Click close, then start it again. Newer versions of Firefox will reopen your tabs from last time. Use Help > About, and make sure your version is 3.0 or newer. (If it's not, click help > check for updates).

Warning: Tab Mix Plus disables this feature. It's own session manager works differently.

Alternatively, just wait until you are finished installing/enabling/disabling extensions to press the Restart Firefox button."

I personally use the Session Manager extension for this. It saves my session, including all open tabs and history, so that when Firefox restarts (or crashes !), I can go back to one of my previous sessions without losing any tabs.