This question is an exact duplicate of:
I don't know whether something's changed here or in https everywhere (for firefox) but I now have to turn it off to log in to meta, otherwise I get bounced to https.tex.stackexchange.com, and when I go back to meta.tex.stackexchange.com I'm still not logged in.
Once logged in I can enable it again.
I don't normally do much more than read on meta so I don't know how long it's been going on - but recent-ish.
(bug tag is speculative I admit)