Provide a way to pick manifest version when compiling

[eko234]

~~Hi, I was trying to run Krabby on firefox, I have a developer edition, but when I try to follow the instructions I jsut get a "This addon could not be installed because it appears to be corrupt.", and I can't find why this is, could you help me out?~~ I created this issue due to this error but now I find it is an issue with Firefox complaining about manifest V3, I worked it around getting the repos and checking out the version prior to the change to V3, I think it would be good to provide a way for the user to pick the manifest version he wants to compile the packages with, doing it manually is no big deal but surely takes time if you (like me) didn't knew a thing about addons and firefox and so on, thanks for you work, I'm enjoying Krabby now.

[em429]

I had the same problem — Firefox is introducing manifest v3 though so might not be a problem for long.