Did you make sure to include the space characters in the same element as the monospace characters because otherwise you may end up with normal spaces that have a different length than monospace space characters. A recent thread you can look at:

• /questions/1412335 1, Strange formatting of <B> inside <PRE>, and 2, Unexpected formatting of bogus <D> element

OK, I tested it and it looks fine for me. Are you using extensions that can block remote fonts ?

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can right-click and select "Inspect" to open the built-in Inspector with this element selected.

• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/open_the_inspector/
• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/examine_and_edit_html/

You can check in the Rules tab in the right panel in the Inspector what font-family is used for selected text. You can check in the Fonts tab in the right panel in the Inspector what font is actually used because Firefox might be using a different font than specified by the website.

• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/edit_fonts/

Thanks for the help, the problem unfortunately remains in troubleshoot-mode aswell. I can also not really imagine that it's a "my machine" problem, since it occurs on 2 of my devices and on the Desktop of a freind aswell (again just Firefox, not Opera or Chrome)

The Issue only occures with the bold Font, thats becasue Roboto mono doesn't supprt my choosen weight of 700. So I switched to Source Code Pro (which does support it), now it works. (view image)

I'd still argue that it's weird that it worked on every OS-Browser Combo exept Windows+Firefox.

Resolved.