Whenever I log into Hotmail (and usually when I log into Gmail) as I begin writing everything is marked as mispelled, and when I click on Language, I find it has defaulted to French.

I do occasionally work in French, and I am used to the system fefulting to whichever language I have used more recently, but since the last upgrade to 34.0, it always defaults to French. If I have started with Hotmail, it almost always taks two or three tries to change the default; with Gmail it usually catches on the first try.

I am relatively sure this is a Firefox bug, as it does not occur if I open with Opera.

Any ideas? Or should I just be passing a bug report to Mozilla?

You've likely done this, but if not, perhaps you could check your dictionaries and languages under Tools/Add-ons, and could check Edit/Preferences/Content and then select the "Choose.." button under languages to see in what order they're listed. Don't know if it'll help, but it could be a place to start.

I'm sorry it took so long to respond to this.
This solved the problem. When I looked, I was surprised to see that Firefox showed only a French dictionary as installed (which seemed odd, as spellcheck was always functioning); I installed an English dictionary,made sure it was the first in line, and the problem immediately went away.
Many thanks!