Add choice of tab unloader

[tranquil-tr0]

Suggestion to expose option between native Firefox tab unloader and Zen Browser tab unloader. This could also be a question, since I can't quite understand why the Zen Browser tab unloader would be the default (maybe to allow for the right-click context menu to show unload option?), so also a question. For context, the option I am referring to can be found in about: config under browser.tabs.unloadOnLowMemory and zen.tab-unloader.enabled. My personal preference comes from the default being responsive to memory use, which I would imagine could benefit others, as evidenced by #3076.

[mokazemi]

I'm having the same suggestion too as a daily user:

Add support for different options and ways of tab unloader (unload after a period of inactivity, unload on low memory, unload manually, etc.)

[tranquil-tr0]

This would make a lot more sense. Since I assume there is some advantage to using Zen tab unloader, if it is approachable the superior solution is clearly to integrate "on low memory" features, etc.

[ABruel]

I need this. Firefox's default tab unloader is a mess. Eats my RAM unnecessarily by keeping a bunch of old tabs open just because I have a lot of free RAM. I don't want my browser thinking it has a right to all of my RAM.

[tranquil-tr0]

This is no longer relevant, as v1.12.9b changed the tab discarder/unloader method to use the Firefox default.