Firefox on Linux currently doesn't support a mixture of HiDPI and non-HiDPI monitors, it has a single DPI setting. I think it will be supported when this [1] bug is resolved.

Shameless plug: I have written an addon to workaround this [2].

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=975919

[2] https://addons.mozilla.org/en-US/firefox/addon/autohidpi/