I would like to see a screenshot of that xfce-b5 theme and what you want to achieve. For reference, this is how Firefox/Iceweasel looks in my system, running GNOME Shell: https://i.imgur.com/VedZNiC.png
I didn't crop it: that is all that is visible when maximised. I'm using tree-style-tabs for Firefox, and the Hide Top Bar and Maximus Two extensions for the Shell.
Most gtk2-based (or dependent, in case of eclipse) apps I use at daily basis (incl. floating deadbeef settings window with older gtk2 plugin) side-by-side. I haven't included evince (gtk3) as I am using partially broken gtk3-Mist theme (not compact as xfce-b5, but still better than Adwaita). Nothing scaled, cropped or such. Btw, I use Classic Theme Restorer plugin, which also helps with firefox UI compactification.
This firefox theme makes the app's decorations really small. The icons look as if they were designed in 1995, but if there is a choice between slick and compact, I pick compact.
On second thought, Classic Theme Restorer works just as well with the latest firefox dev build.
TreeStyleTabs, like felipeerias already mentioned seems to be THE reason why a whole lot of people use Firefox. (Others like me have a bunch of additional reasons.)
I didn't crop it: that is all that is visible when maximised. I'm using tree-style-tabs for Firefox, and the Hide Top Bar and Maximus Two extensions for the Shell.