I've had this issue too. Firefox doesn't seem to load the whole dom and hence once search in what it has loaded in memory at the time. This is very common when searching through large pdfs. Which is crazy considering how much RAM it hoards. wtf is it doing with all the RAM? Load the whole document god damn it.
@kensp yeah! How is it taking so many MB of ram all on it's own, yet it can't cache more than three pages of text at a time?
Linux geeks doing what Linux geeks do...