I just updated to Firefox v57 in Linux Mint (sarah), package version labeled "57.0+linuxmint1+sylvia", and now a number of pages won't display text, or display non-printing character indicators. I can turn off "Allow pages to choose their own fonts" and reproduce the problem for some fonts, going down the list of fonts in the default font setting. In particular, Andale Mono, Arial, Arial Black, Comic Sans MS, Impact, Times New Roman and probably several more won't display. I note that most, perhaps all of the problem fonts are Microsoft TTF fonts, although these fonts work fine in other applications. I don't know if this is a bug, or a configuration issue, or a policy decision. See the page snapshot of a Google result for the term "python".

The workaround is to use a default font which _will_ display and not allow pages to select their own fonts, which is a hobble on FF, and I'd rather not have to fall back on it.