I haven't had a single issue with Firefox (54) out of the box. I know that some add-ons can break the multi-process windows feature, but out of the box has been fine, both with clean VirtualBox installs and bare-metal desktop installs.

As far as I can see, that's a configuration- or user-specific problem. I haven't seen any reports. Firefox (or, more technically, "the default browser") must work in order to meet our release criteria.

Haven't experienced any problems so far.