I asked about a possible solution to this elsewhere, generating no response as yet: https://mail.gnome.org/archives/gtk-devel-list/2016-July/msg00028.html https://lists.fedoraproject.org/archives/list/kde@lists.fedoraproject.org/th...

Apparently, a fix from mozilla.org is unlikely forthcoming, while a particular Gnome commit is highly likely responsible for the regression in Firefox and SeaMonkey built by mozilla.org with GTK3 toolkit: "UI text sizes no longer inherited from Linux system" https://bugzilla.mozilla.org/show_bug.cgi?id=1269274

Screenshot of mozilla.org's FF48 in TDE on openSUSE 1920x1080 @132DPI display showing the problem: http://fm.no-ip.com/SS/Moz/uifontsRV48-s422tde-132.jpg

Screenshot of mozilla.org's FF48 in TDE on openSUSE 1680x1050 @108DPI display showing the problem: http://fm.no-ip.com/SS/Moz/uifontsRV48-s422tde-108.jpg

Screenshot of mozilla.org's FF48 in TDE on openSUSE 1680x1050 @108DPI display showing no such problem before the update of GTK3 from 3.16 to 3.20: http://fm.no-ip.com/SS/Moz/uifontsRV48-s421tde-108.jpg

Screenshot of mozilla.org's FF48 and FF45 on Fedora 24 with Plasma5 @168DPI display showing problem is not unique to TDE, and doesn't exist in FF45: http://fm.no-ip.com/SS/Moz/uifontsRV48-f24K5-168.jpg

Mozilla.org's FF ESR 45 is still built with GTK2, so doesn't have the problem. SeaMonkey 2.40 is also built with GTK2, so also has no problem. All the mozilla.org alphas and betas are being built with GTK3, so all manifest the problem unless used with system GTK3 libs 3.16 or older.

Whether this problem shows up in Firefox or SeaMonkey provided by individual distros depends on which toolkit is used to build them. In Fedora, the switch to GTK3 has been made, as have been the GTK3 libs past 3.16, the last before the upstream patch. Thus, all current rpms from Fedora produce the problem. In openSUSE Tumbleweed, Firefox 48 is still built with GTK2, so even though it has switched to 3.20 GTK3 libs, the problem there hasn't yet appeared. Stretch has 3.20 GTK3 libs, builds FF 45ESR with GTK2, so has the problem in FF with mozilla.org builds but not with its available FF .deb package. Jessie is OK, with only GTK3 lib 3.14. Buntu 16.04's GTKs3 lib is 3.18. Its FF 48 buildconfig screen doesn't include the string gtk anywhere, but apparently it's using the GTK3 default, and thus also not obeying TDE's UI font specification.

So, to eventually restore parity to all distro and version installations, always having the Geckos use the UI fonts specified in Trinity desktop settings, something needs to be done. The question is where. Is getting upstream patch reversion the right answer? Would some new or altered gtk-*qt*-trinity package be good, only, or better answer? Would some new or altered theme package be a right answer? Is there already some solution that just hasn't been installed yet?