hi bernardwi89, 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'

Hello bernardwi89,

In addition to the above :

Just in case you have a language pack installed, you should pay extra attention to the last few posts in the bug report that philipp mentioned.

'firefox.exe -purgecaches' didn't help for me. but going to my Extensions and themes, I found that the English US language pack was incompatible with my Firefox version, so I removed it and added the language again.

Quote: I found that the English US language pack was incompatible with my Firefox version, so I removed it and added the language again.

A check for updates via the cogwheel button on the about:addons page would probably had worked as well.