Bump to newer selenium-firefox custom image arrived with surprises

Recently, to fix a bug that was happening in our custom selenium-firefox image, we bumped it to current one (newer underlying OS, newer xvfb, same selenium version and same old firefox version).

That has come with some undesirable problems. Namely:

• (master) : Passes (sometimes fails in "Award badge on course completion")
  https://ci.moodle.org/view/Testing/job/DEV.01%20-%20Developer-requested%20Behat/2266/

• (37_STABLE) : Fails (6 different login failures)
  https://ci.moodle.org/view/Testing/job/DEV.01%20-%20Developer-requested%20Behat/2267/

• (36_STABLE) : Fails (6 different login failures)
  https://ci.moodle.org/view/Testing/job/DEV.01%20-%20Developer-requested%20Behat/2268/

• (35_STABLE) : Passes
  https://ci.moodle.org/view/Testing/job/DEV.01%20-%20Developer-requested%20Behat/2270/
  (but there is a new failure @core_backup)

Also, we have tried rebuilding our custom selenium-firefox package with many different upstream selenium-firefox versions and the conclusion is that the change to 3.141.59-europium as base was the problem, and that there isn't any version fixing that:

Running @core_course,@enrol_self,@mod_quiz (36_STABLE - 20191008)

3.141.59-bismuth  : https://pastebin.com/Z8KkvyDR (1 quiz failures)

...

3.141.59-dubnium  : https://pastebin.com/uTZvyGTm (1 quiz failures)

3.141.59-europium : https://pastebin.com/RFt7u6TY (6 login failures)

...

3.141.59-oxygen   : https://pastebin.com/qVByP8Ly (6 login failures)

...

3.141.59-selenium : https://pastebin.com/Db731XJE (6 login failures)

...

3.141.59-vanadium : https://pastebin.com/Bt3Kyv5H (6 login failures)

LOCAL RUN         : https://pastebin.com/eWesa8qh (passes)

So this is about to try to understand why those, constant, failures are happening and try to kill them to be back to green with firefox (hoping that, at some point we are able to bump to marionette / W3C driver, grrr, but that's apart).