I stopped to use Firefox and already posted this question from inside Safari, but I was asked to post it from inside of Firefox (by Pkshadow) that it can help developers to define the exact problem. So, here it is. After Firefox update I started to notice that icons for all my files changed on my Mac and they start to open only with Illustrator!? Even DOC files! When I change setting back through Finder File Info it stays normal for short while but soon changes everything to Illustrator opening again! Only solution is to close Firefox and restart a computer. Then all goes to normal and files have icons associated with their programs and open as needed. But if I open Firefox it changes everything to Illustrator opening again! How and why Firefox access and change BASIC settings on Mac OS? It is not good if it is even possible. I like Firefox and have a lot of bookmarks on it but cannot use it anymore! I searched online and there more complains about this problem. This problem is common for Mac users with Firefox, and it is definitely a Firefox bug. Please, look at these forums, especially the first one - https://discussions.apple.com/thread/8142156 https://forums.adobe.com/thread/2412747?start=40&tstart=0 https://forums.adobe.com/thread/2412747 https://discussions.apple.com/thread/8233783 https://forums.adobe.com/thread/2405260#2405260 People usually start to blame Apple and Adobe, only a few guessed that it is a Firefox bug. When Firefox is closed and a computer is restarted everything goes back to normal. Pkshadow gave me a list of instructions how to reinstall Firefox to try to solve this problem. Unfortunately I am kind of very busy and was not able to do it, may be later, when I have some time for that kind of intervention. For now I am using Safari, although, as I already said, I like Firefox more. I really hope that maybe the next Firefox update can somehow remove this bug. I hope that this post may help to solve it. Thank you!