Tab Ceiling per brp

This is a very simple add-on I made to get my tabs under control. It does four things:

1. Allows you to set a tab ceiling, above which new tabs will be blocked from opening. (Android: just block new tabs from opening, since Android add-ons have no way of counting open tabs).
2. Allows you to change links so that they won't open in new tabs. This can be turned on in settings.
3. Distinguishes between old and new tabs, to prevent from closing tabs as they reopening after inactivation (on Android) or after a session restore (on Desktop).
4. Provides a settings pop-up to allow configuration of each of the above behaviors.
5. New in 1.6.0: Provides a grace count button, so that you can easily allow a single (or small number) of new tabs for those sites which need them to function (OAuth, etc).

Usage:
1. Install the add-on
2. Open the add-on settings widget from the toolbar
3. Turn on the ceiling (Desktop) or the blocking (Android)
4. Configure number of tabs to limit to, and whether links should be overridden.
I personally prefer the config that links are not overridden, because I'm afraid of losing state when the override unexpectedly opens a link in the current tab.
5. Enjoy!

On FF Android, addons are unable to access the current number of open tabs, and so the toggle just blocks opening new tabs.

As of 1.6.0, FF Android occasionally closes inactive tabs when they are loaded! This could lead to data loss! Use Android version at your own risk!

The add-on seems to behave correctly on FF desktop. That said, if you test it and find an issue, please file an issue on github.