I would say grab your profile, https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data and toss it all and start with a new compile for ver 59 with a new Swedish Interface. Can you use a compile from https://www.mozilla.org/firefox/all/

I have downloaded Swedish 64-bit version from https://www.mozilla.org/firefox/all/ and run it from the folder without installing it. Same thing happens, i.e. no Swedish menues etc. Why does this happen at all? All previous updates has kept the Swedish UI.

Now there is an update to 59.0.1 and if I download it from https://www.mozilla.org/firefox/all/ and run it from the folder without installing it, it actually have the Swedish UI now. But still crash when visiting www.tradera.com. However, when I update the installed 59.0 to 59.0.1 it still has the English UI, even when the Swedish language pack also is updated to 59.0.1. But by copying the downloaded files to (and replace) usr/lib/firefox I got the Swedish UI back.

Hi, could you report this : If you have a bug, file a bug report. https://bugzilla.mozilla.org/ Bug Writing Guidelines : https://developer.mozilla.org/en-US/docs/Mozilla/QA/Bug_writing_guidelines