I had this exact annoying issue for a long time with font display in Safari (Firefox was fine)... and I think I've finally fixed it.
I also use Suitcase Fusion 3 for my Adobe Creative Suite apps and initially I found that if I reboot in Safe Mode to clear the font cache OR use Suitcase to clear the font case it would provide a very temporary "cure".
I use a lot of fonts mostly managed by Suitcase Fusion 3 and on a hunch I went through the fonts I constantly use and cleared and deleted all the Postscript fonts. Where needed I substituted with OpenType or TrueType fonts.
PostScript is an older font format and I thought it might be an issue.
Happily I can report that while before many web pages, including my banking pages, displayed type as "Keyboard A's", now I have not had a single font issue with any Safari page.
In my case, at least, I believe that not using any PostScript type fonts solved the problem. Use OpenType or TrueType.