Hey, remember me? I'm still around, but the instructions on this site have held up over the years so there was no need to update them. That changes now with Firefox 48. Starting with this version, extension signing is enforced with no way to turn it off in user.js (well, at least in the Firefox-branded release and beta channels anyway.) Earlier versions would let you off with just a warning if you set xpinstall.signatures.required to false, but that option is ignored now.
I updated the extension to MSI instructions, and the only significant change was not to alter <maxversion> in install.rdf anymore. Firefox now assumes extensions are compatible with future releases by default so there's no need to. More importantly, changing install.rdf would probably invalidate the signature (I haven't tested it though) so we can't do that anymore. Fortunately, repackaging the XPI as a MSI does not. I've successfully installed both Flashblock and IE Tab v2 this way and Firefox still recognizes them as signed.
This is a Part 1 because I still have to work on instructions for getting an altered version of XMarks to work. If it's possible, it will involve creating an unlisted extension at addons.mozilla.org so it can be signed. Stay tuned.
Although this is technically a blog, it's primary content is a series of articles on how to get Firefox working in a corporate Windows environment. Later ones build on earlier ones, so you might want to use the Table of Contents on the right to read through it chronologically instead of reading straight down from here.