Convert to web extension

[est31]

Firefox 57 and later will only allow to load web extensions [1][2]. This add on is currently a legacy extension, and Firefox will refuse to load it starting with Version 57. Therefore, this extension needs to be ported [3].

Current nightlies of Firefox already only accept to load WebExtensions [4].

[1]: https://blog.mozilla.org/addons/2016/11/23/add-ons-in-2017/
[2]: https://bugzilla.mozilla.org/show_bug.cgi?id=1336576
[3]: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Porting_a_legacy_Firefox_add-on
[4]: https://bugzilla.mozilla.org/show_bug.cgi?id=1388946

[Norbert (nrbrtx)]

Two options here - make official Firefox ESR package (see bug 1676164) or convert xul-ext-ubufox to WebExtension.