This looks as if it would be useful but do not use it until someone with particular experience is happy to verify it; and they may supply an alternative. http://www.quicklyuninstall.com/article/Quickly-Uinstall-Anti-Aliasing-Tuner-11.08.18.01.html Your system details actually show Fx 24.6. Is that a different computer?

I am unshure to do that. Maybe the instructions cause the same problem as simply disabling the add on. And then my profile is completely lost.

Regarding the 24.6, I went back to the LTS Version until I know what to do.

I have found a part of the problem.

If I set

gfx.content.azure.enabled

to false, the charakters are gone.

As I can see this setting is used to disable the anti-alasing for the font in firefox. Is there another way to disable the antialasing? The text in my firefox looks with antialasing quite washy.

I meant

gfx.font_rendering.cleartype_params.rendering_mode

The thing is, I have deactivated cleartype on my windows and I also don't want anti-alasing in my firefox.

Try to create a new Boolean pref on the about:config page via the right-click context menu with the name layout.paint_rects_separately and set the value to true and leave hardware acceleration enabled.