Support Firefox 56

[indygreg]

My Nightly just upgraded to version 56 and Tab Center stopped working.

I lost my vertical tabs and my productivity suffered. Tab Center
appeared to work just fine with yesterday's Nightly. So it should
still work with Nightly 56.

According to issue #1102, a replacement doesn't yet appear to be ready.
And Tab Center is still being advertised at
https://testpilot.firefox.com/experiments/tab-center/. So it seems
prudent to still support Tab Center with Firefox 56 so users aren't
annoyed.

[bwinton]

As mentioned in another bug, Nightly is changing too much and too quickly for TabCenter to keep up. And it would be a very brief respite, since we're killing SDK-based add-ons in 57 anyways. We may enable 56 support when 56 lands on beta, depending on how different it is from 55's beta, but that's as good as it's going to get, I'm afraid.

Instead of trying to keep this add-on alive for another six weeks, I've been suggesting people spend their time contributing to https://github.com/eoger/tabcenter-redux/ instead, because it should work in all future versions of Firefox.

Sorry, Greg.

[indygreg]

I'm fine with switching to another add-on. But, how do I install such an add-on? I don't see a release of eoger/tabcenter-redux on AMO. It is somewhat user hostile to tell me (and the ~128k other users of Tab Center according to https://testpilot.firefox.com/experiments/tab-center) to use something that can't (apparently) be installed via official AMO mechanisms today.

[bwinton]

tabcenter-redux is still in progress, and isn't ready for AMO yet. It's something people will have to help build, not a drop-in replacement. (Which is why I'm only recommending it for people who comment on github, not in Discourse. After thinking about it a little, you could also build your own copy, and install that. I think nightly still respects the xpinstall.signatures.required pref, so you wouldn't even need to get it signed, and I'm pretty sure we're not going to release another version of Tab Center, so you wouldn't have to worry about being overwritten with a newer version…)

I certainly don't mean to be hostile, but it seems that a lot of people are forgetting the experimental nature of stuff in Test Pilot. We're explicitly not committing to maintaining experiments indefinitely, or providing replacements when stuff goes away. I agree it kinda sucks, but that's sort of what you're signing up for when you agree to test experimental features. Heck, I'm still searching for a WebExtension-based TabGroups replacement, and that was a regular add-on, not an experimental one!