No, it is going to break after every minor update. Most legacy extensions already don't work anymore in Firefox Nightly, even if you tell it to load them [0], s

No, it is going to break after every minor update. Most legacy extensions already don't work anymore in Firefox Nightly, even if you tell it to load them [0], simply because Mozilla has very much already used their new freedom to throw out tons of legacy code [1] and to refactor what's left. The Mozilla-supplied legacy extensions continue to function, because Mozilla is updating them as they make changes to the internals of Firefox. So, I imagine maintaining such a legacy extension is now as much work as it is to just directly contribute and maintain the feature in Firefox's codebase. [0]: "extensions.legacy.enabled" in about:config. This won't be shipped in Stable, though, and will likely be taken out from Firefox Nightly soon or with the release of Firefox 57. [1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1347507 https://bugzilla.mozilla.org/show_bug.cgi?id=1347507