We have a font tag too, so I think the issue here is that the font tag there seems to be able to include a font-family style string, which we don't recognise.
Should be simple enough to fix, though.
Use the following add-on:
https://xenforo.com/community/resources/post-content-find-replace.1549/
It allows you to do regular expression searches on posts throughout your forum and make replacements.