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.
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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