On my WINDOZE desktop, CYBERFOX remains as my DEFAULT BROWSER. As I see it, it has all the advantages of a stable FIREFOX, but for some reason, it is a LOT quicker to load than 'the real thing'. But whenever a new version of 'the real thing' gets a general release, I always give it a fair trial, in the hope that they've improved its notorious sluggishness once and for all. When version 56 arrived, I was delighted to find that many of the problems I'd had with earlier 64-bit variants had been cured. It's a LOT quicker, in fact it's pretty much caught up with its 'FORKED OFF' little sister. I was on the verge of making 'the real Firefox' my DEFAULT again after all these years. But then I found that I couldn't log into TWITTER on the new beast.
Now Twitter doesn't really like you using PRIVATE BROWSING. It especially doesn't like it when you use any sort of TRACKING PREVENTION. That is why I get daily email notifications telling me that there has been an 'UNKNOWN LOG-IN' to my account. The poor little twitbots just can't recognise you as a real person! I get so many of those in a day that they are forwarded straight to the bin now! Naturally, when I try out a 'new' browser, I immediately tweak all the 'SECURITY & PRIVACY' settings, so that I know nobody is looking over my shoulder all the time and that any 'HISTORY' will get flushed away when I close the window. My ideal internet browser would be a Windows or Linux version of Android's FIREFOX FOCUS, which does all that for you at the click of a single button.
On finding that Twitter "WOULDN'T WORK" on the latest Firefox, I systematically went through everything to track down what it was objecting to:- I disabled plug-ins, including my beloved STYLISH CSS scripts and the 'HTTPS BY DEFAULT' I'd installed quite recently (I find that if something suddenly doesn't work, one should always retrace one's steps to the most recent changes). I tried a number of DIFFERENT combinations of 'Fox's own 'Privacy' settings, including disabling the 'DO NOT TRACK' completely. I unblocked the blocked cookies... Then I found it! I UNTICKED THE "BLOCK POP-UP WINDOWS" OPTION
UPDATE: You can leave the "Block Pop-Up Windows" option ticked, but add the Twitter address to its 'EXCEPTIONS' list. It seems funny that I've never had to do that with any previous versions though !?!
(well, sort of...) Now I don't know if this is a bug in the new Firefox or changes to Twitter itself. I rather suspect the latter, whenever issues of 'compatibility' crop up. A Twitterchum tells me that there is a version 56.0.1 of Firefox already out there (for those who receive the 'bleeding edge' releases) so it MIGHT be that they've found some bugs to fix already. I find I rather like the latest version of FIREFOX. They're getting there, but I don't know if I'm QUITE ready to go all in just yet...