Document future of Greasemonkey support

[reitzig]

As I'm sure you're aware, Greasemonkey 4, due to release with FF 57, is not backwards compatible. Scripts will need to be adapted.

Please document which of the following options for Greasemonkey <4 users will be open regarding SOUP:

1. SOUP will be ported to Greasemonkey 4.
2. SOUP will no longer be support Greasemonkey; use Tampermonkey (or...?).
3. SOUP will no longer be support Firefox as a userscript; use the browser extension.

[vyznev]

As far as I'm aware, SOUP should in fact be compatible with GM4 as it is, since it does not use the GM_* API or unsafeWindow. That said, I should indeed test that and document the results, since I'm sure you're not the only one with that question.

[vyznev]

I've tested SOUP with GM4, and it indeed works just fine, except for a few issues caused by known FF57 / GM4 bugs. I've worked around most of those issues, although the bug that prevents script installation from GitHub is not something I can do much about. (I could provide an alternative installation source for GM4 users, but it's probably easier to just recommend the stand-alone version of SOUP instead.) Anyway, the SOUP StackApps page already describes the current state of GM4 support, and I expect that, once GM4 stabilizes properly, it should continue to run SOUP just fine.