max-width and images [M975632 et al]

[classilla]

Uplifted from https://tenfourfox.tenderapp.com/discussions/problems/8581-bad-scaling

https://bugzilla.mozilla.org/show_bug.cgi?id=975632
other tests: https://bugzilla.mozilla.org/show_bug.cgi?id=434230 and https://bugzilla.mozilla.org/show_bug.cgi?id=441046

The meat of the patches is in https://bugzilla.mozilla.org/show_bug.cgi?id=823483 but we need to also go through all the potential regressions listed.

[classilla]

Since we have to have a second FPR7 beta due to #495 let's go ahead and throw this in.

[classilla]

Original reporter's case and all linked bugs on M823483 now pass as expected. However, it would be nice to solve the flexbox case at the same time for height since we're making these other changes. That appears to be https://bugzilla.mozilla.org/show_bug.cgi?id=1030952