As archlinux could be run almost as minimalistic as you wish, you don't really need to have some default application handler and you can do it on your own. Priority of default application selection is described in archlinux wiki. If no program is set as default for specific event, order from /usr/share/applications/mimeinfo.cache is used.

as C is third in alphabet compared to F and there is to my best knowledge no NoScript in chromium, some changes should be made. By directly editing mimeinfo.cache mentioned above, you can easily confirm that no other configuration list is in operation (just changing browser mime on first position in this file - fe change chromium to firefox or vim or anything else.. - don't forget to put it back when you're finished), if change of default browser occurs, there is no mimeapps.list which manages considered action. Than it's easy task to just edit your ~/.config/mimeapps.list accordingly:

[Default Applications]
...
...
x-scheme-handler/http=firefox.desktop;
x-scheme-handler/https=firefox.desktop;
text/html=firefox.desktop;
...

So far I did not fully understood communication between mutt and mimeapps.list, it interacts, but not fully (for example .pdf is recognized without any issue whereas excel files are having a hard time). Generally robust way how to link extensions with default apps is by using mailcap file as shown here: https://github.com/jelly/Dotfiles/blob/master/.mutt/mailcap (needs to be called in .muttrc) and described here: https://gitlab.com/muttmua/mutt/wikis/MuttFaq/Attachment

I could not get used to read pdfs in gimp…

x-scheme-hanlder/pdf=mupdf.desktop;
application/pdf=mupdf.desktop;