I've finally figured it out myself.
Booting my system in recovery mode and deleting the file /home/<myUser>/.config/xfce4/xfconf/xfce-perchannel-xml/xsettings.xml has returned Firefox's UI to its default state after I've rebooted normally.
EDIT: turns out it was a problem with the font I had set, changing it solves the issue without deleting any files.