I can't see checkboxes in Firefox anymore. They are there, I can check them but I can't see if they are checked or not.
I already found some hints that it's caused by GTK3 but I didn't find a usable solution.

Anyone with the same problem or even a solution?

Have you tried another gtk3 theme? And which particular check boxes are these? Are they "internal" to Firefox such as in about:preferences and such or are they on web pages?

I switched the theme but it didn't help.

The extern checkboxes. So in the firefox preferences I can see the checkboxes for preferences but when I register here I didn't see the checkbox for accepting the rules.

For example this website looks like:
https://faculty.washington.edu/chudler/java/boxes.html

tomtoto wrote:I switched the theme but it didn't help.

The extern checkboxes. So in the firefox preferences I can see the checkboxes for preferences but when I register here I didn't see the checkbox for accepting the rules.

For example this website looks like:
https://faculty.washington.edu/chudler/java/boxes.html
...

When I visit that site from Kubuntu 16.04, I see

Test your skill. How many boxes can you check in 20 seconds? (IE Explorer Only)

as well as a 10x10 grid of checkboxes. The site seems to work for me even though I'm using Firefox.