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"
Just close the browser and then re-open it.
You can also use this addin QuickRestart, which will add a button you can put on your toolbar to allow for 1-click restart.
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.
Ubiquity can do this, among it's many, many features, with "restart-firefox" :)