According to bug 1714353 in the Mozilla bug tracker the issue seems to be ultimately caused by incorrect color depth handling in Fluxbox (see discussion here). The root cause apparently has been fixed upstream, but the patch doesn't seem to have made it into the Debian Fluxbox package yet.
If you don't want to recompile your window manager you can try mitigating the issue on the Firefox side. In
about:config set these two options:
Alternatively add the following two lines to
user.js in your Firefox profile:
Then restart Firefox. That resolved the problem for me.