Darkoshi

I totally get it being "more of an amusement than an actual problem" but the investigator in me has to track this stuff down, and the fact that you say web pages in general seem to be rather randomly in play and that it might be a browser (Firefox) problem intrigues me. Have you considered checking through about:config for any language flags? Add-ons tend to set a lot of their preferences in such a way that the results of those settings are passed to Firefox so about:config can reveal them. In fact... *plays in about:config for a minute* a search on the word "language" should turn up a couple of flags; so should a search on "en-us".

It would be nice to know just to satisfy my now-burning curiosity how this turns out, if you manage to track down the source of the bug/problem/issue.