Often when I open documents in the built-in PDF viewer Firefox freezes for a few seconds and then when it returns to life all the text not associated with a particular webpage is gone. The tabs for the Firefox windows have no text. The search bar has no text. The address bar has no text. The bookmarks bar has no text. If I right click I get a menu with no text. If Firefox opens any windows (such as below when I click "Automatically add") those windows have no text. I have a screenshot of this phenomenon but don't see any way of attaching it. Restarting Firefox only occasionally resolves the issue.

This was happening while I filled out this form and collected the troubleshooting information below (by guessing which textless window buttons did what.)