Comment # 5 on bug 1076907 from Wolfgang Rosenauer

I don't think that it is very useful to set Firefox as a mimehandler for
image/jpeg. Even a stripped down system I would expect to have an image viewer
which is more more lightweight than Firefox.

Still what you are seeing why Firefox is not set as mimehandler for text/html.

I cannot tell why or if zypper should pick up those provides from the desktop
file but I'll investigate.
So far mime-types are read from the desktop file and exactly should be
registered via the desktop file utils.
For that Firefox has the following in %post:

%post
# update mime and desktop database
%mime_database_post
%desktop_database_post
%icon_theme_cache_post
exit 0

As said I'll investigate what the expectation is about RPM provides but to my
understanding Firefox provides everything and also calls out to update the
databases. Why it's not working in your case might be an effect of something
else?

Looking at my /usr/share/applications/mimeinfo.cache everything looks fine.