Just FYI: after last update 25.0.1 my Firefox was transformed from Polish to English
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Were you using the PA.c Platform to update it and did you have it set to English at the time? If so, it will upgrade Firefox to match the platform.
Sometimes, the impossible can become possible, if you're awesome!
Before update Firefox language was set to Polish and FireFox.
Update via standard PA.c update.
There is a problem with FireFox itself or polish lang-pack.
Polish language pack (https://addons.mozilla.org/en-US/firefox/addon/polski-language-pack ) is not working too.
And, interesting, I've lost polish dictionary too. Reinstalled dictionary works fine.
It is the first such case since many years.
I have no need do dig deeper, IMO it is a FireFox' problem, not PA.c