Since the quantum update all AddOns which allowed to change key bindings in Firefox seemed to have stopped working/being supported.
Is there a method to change the default key bindings in Firefox Quantum?
There is a way. It's not super official, but basically you can unpack
browser/omni.ja, edit the keybindings in
chrome/browser/content/browser/browser.xul, repack it, delete startup cache and it will work.
Alternatively, you can compile your own firefox and then you don't need to unpack the binary, if you consider unpacking and repacking more hacky, than building.
Another advantage of building is that you can store your modifications on top of the official sources in git and always rebase, like I do here: https://github.com/errge/gecko-dev/tree/gregzilla-patched-20181223
I advise you to first start with the binary option, because you will have working keyboard shortcuts in 20 minutes, instead of just being at the start of the mercurial clone procedure :)
Both of these methods are independent of any extensions/webextensions and will ALWAYS work, even in the location bar and even on protected pages (as you asked in the comments). So they will work better than remapping webextensions.
I have an article written up with all the details that may interest you: https://github.com/nilcons/firefox-hacks
If you have more questions, please report issues on github.
From Firefox support: https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly :
Note: You can customize keyboard shortcuts for Firefox using the https://addons.mozilla.org/firefox/addon/saka-key/ extension.
I hope it is what you were looking for.
I am not sure what kind of shortcuts you are after but there is a great extensions called Vimium FF that allows you to browse completely mouseless. This extensions offers a filter mechanism that allows you to decide in which websites the keybindings should work.
It is still in experimental stage, since it is a port from a Chrome extensions but I am using and did encounter any bug or problem.
The bindings follow the VIM bindings and should be natural if you are already familiar with those, otherwise, you can custumize them to your own taste.