I am in the US and would like to use en_US. However, some time (perhaps on the scale of weeks -- perhaps related to Firefox upgrades?) after setting my language to en_US, I notice that Firefox is suggesting adding 'u' to words where it doesn't belong, such as "behavior". When I check the languages listed on the context menu, I see that "English (Zimbabwe)" is selected.

I just replicated this over a simple quit and restart of Firefox.

Firefox has reverted again :(

Does it happen with recent versions?

No, but I've since switched to a Linux desktop (originally reported on a Mac), so that may not be useful data.

Hah, and once I said that, I'm alerted that "artifact" is in fact spelled "artefact" in English (Malawi). firefox-38.0.5-1.fc20.x86_64

I'd say that's fixed by bug 717433 in FF 43.

As I said, I believe this is fixed. Also I suggest to uninstall en-ZA (which is South Africa) if you don't want it.

I'm not sure how en-ZA was relevant here, and at any rate it's not separately installed. But yes, I haven't seen this in quite a while, so presumably this was FIXED (no INCOMPLETE) by the bug you referred to.

As far as I can see "English (Zimbabwe)" does not exist as FF dictionary, neither does "English (Malawi)". Closest locality: South Africa ;-) "Incomplete" since we don't really have sufficient details, no reproducible case and much confusion about which dictionaries are really installed or where Zimbabwe or Malawi came from. If you prefer, make it a duplicate of bug 717433. To mark it "FIXED", I'd have to supply a target version, etc.