AltGr (the right alt key) essentially translates to ctrl+alt on Windows and some Linux configurations. It also acts as a compose key on some configurations, making it quite unreliable for navigation as the system IME may intercept the key before sending it through.
You can rebind it back to a normal alt key at the operating system level if you don't care about the AltGr functionality, that'll probably fix a whole bunch of unexpected problems you may be having in other programs. I think Firefox altering the ctrl+alt+left/right behaviour would break more workflows than it fixes, to be honest.
I forgot what exactly broke, but I do remember assigning something to my ctrl+alt+left/right keys and finding out that various applications (and I think maybe the rich text edit field?) got very confused.
Is this an AltGr thing? On my (US, in Linux) keyboard right-alt-left-arrow works fine, and I'm pretty sure I do the same thing in Windows (again, US keyboard layout).
You can rebind it back to a normal alt key at the operating system level if you don't care about the AltGr functionality, that'll probably fix a whole bunch of unexpected problems you may be having in other programs. I think Firefox altering the ctrl+alt+left/right behaviour would break more workflows than it fixes, to be honest.