Thanks a lot wicknix. I was a bit in doubt that it was related the user agent, but I tried your suggestion — didn't work, unfortunately. If I understood you well the user agent string is not to be entered in about:config, but used as explained now, right?
User agents. A while back because of the Deezer issue I had installed the User Agent Switcher (0.7.3.1) addon, and created, or rather grossly cloned, these user agents:
1. Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:56.0) Gecko/20100101 Firefox/56.0
2. Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:58.0) Gecko/20100101 Firefox/58.0
3. Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36
4. Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; Trident/6.0)
Shortly before I ArcticFox was revealed to me, Deezer had been announcing that they were about to update their site by putting up a floating warning message on accessing the page, saying that my Fx and my other browsers would soon be obsolete. When I started accessing it with AF, that warning went away with the first three user agents above (not no. 4). But that didn't help the fact that Deezer wouldn't play in AF — whereas it was still playing as it always had in Fx 49.5 when they finally upgraded a few weeks ago.
[Btw, your suggestion <Mozilla/5.0 (Windows NT 6.3; WOW64; rv:60.9) Gecko/20100101 Firefox/60.9> doesn't activate playback in Deezer either. The ridiculous thing is that it works perfectly fine in the clumsy interface of VirtualBox/Lubuntu/Fx65.]
Anyway, after this I went on shifting a bit haphazardly between 1, 2 and 3, but lately I have pretty much stuck to 3. — mostly in the vain hope that I would be spared the captcha-hell-by-google. I mean, if Chromium 49 easily takes away the captcha blues, why not Chrome 70? But as said… vain hope.
I don't understand much about how sites interpret this user agent thing (could be that what I concocted there isn't in the correct format), but I also suspect that some sites can detect that they're just that, virtual user agents, not?
Back to the bank, now. I was able to sign in again in Fx 49.5, "only" a question of cookie madness. But after several hours of fuddling about, no amount of cooking or washing away cookies, or turning off ABL, or examining the guts of uBO, or anything else, has got me anywhere in AF.
Here's the sequence, based on signing in Fx 49:
1. launch "
https://secure.mybank.com/xxx/x/xxxx/xxxxxxxx/x/login/index"
2. blank page briefly showing a menu in the middle
3. page turns completely blank for a couple of seconds
4. page — with same URL as above — now shows some options for 2-way authentication
5. option selected leads to external site
6. 2WA
7. back again to original site, 2. above
8. page opens on account, same URL still
In AF, the process stops at 3. The page remains completely blank and the progress bar churns forever. I only wish I knew what it is trying to access that it can't, but where to look?
A bit later: if I access the 2WA page at the external site, point 6., it brings me to 7./2., then I'm stuck again on 3.