Firefox Quantum = Firefox 57, or more precisely, Firefox 57.0 is the "
first of several releases we’re calling Firefox Quantum" (
link)
As noted by others, Firefox 57
breaks lots of add-ons etc. Mozilla have "
transitioned to a new framework" for add-ons with the result that "
only extensions built with this new technology will work in Firefox [Quantum/57]" (
link)
If you are on Firefox 56 you can go to
about:addons and it will highlight any add-ons that won't work with Firefox 57 as "LEGACY". In my case it's 9 out of the 14 add-ons I have installed. Some are inconsequential, others less so.
In particular is
ReminderFox which is the best (for me at least) calendar I've seen, fitting exactly my needs including syncing across all my devices. However, the ReminderFox developers have said they're not going to update it for FF 57 and have recommended users look for other solutions, saying, "
The codebase is quite complex and very UI dependent, so most of it would need to be rewritten entirely. As this is a passion project done in our spare time, we don't have the time currently to do a complete rewrite" (
link) which I'm sure is the case for a lot of add-ons.
So, my recommendation to all Firefox users (assuming you are currently on FF 56) is to, firstly, switch off auto updating of Firefox (and add-ins), and then go to
about:addons and note the ones marked as "LEGACY", and look into those to see if their developers will be providing a FF 57 compatible version. And if not make a decision as to whether you care or not.
If there are no-update add-ons that are important to you the options seem to be:
a) upgrade to Firefox 57 and use
Legacy Extensions to try and find a compatible replacements (
link), and just lump it if you can't.
b) keep auto-updating turned off and stick with Firefox 56 indefinitely (or at least until suitable replacements become available)
c) use Firefox ESR (Extended Support Release) which is basically FF 52 but with all (and only) "
high-risk/high-impact security vulnerabilities" fixes since, until June 2018 when it too will leapfrog to Quantum (
link)
d) install
Pale Moon, which is a lightweight and fast browser "
forked off from Firefox/Mozilla code a number of years ago" and, importantly, runs Firefox add-ons.
I'm trying the latter, Pale Moon, and AFAICS it's identical to Firefox, is pretty zippy, and runs all of the add-ons I've installed so far ... all first impressions as I've only just started trying it ...