My preferred language is English, but any context menu is in Swedish of all things. And some websites decide to be in Swedish entirely, for some reason.
hi mgnslnd, mozilla's developers are looking into this issue in this bug report.
in the meantime, the following steps are reported by other affected users to solve the mixed language display in the UI:
- close all running instances of firefox
- press windows-key + R to open the run dialog
- open: 'firefox.exe -purgecaches'