This may help; Keybinder {web link} Customize, override or disable almost any shortcut. Based on the (discontinued) Customizable Shortcuts add-on by Tim Taubert.

Firefox has some built-in settings related to "accelerator" keys like Command, Ctrl, and option/alt, but I'm not quite sure what happens if you change them. If you feel like experimenting:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste ui.key and pause while the list is filtered

(3) You can experiment with the settings (to reset to default, either right-click or Ctrl+click to call up the context menu and choose Reset)

(A) Accelerator key for keyboard shortcuts

ui.key.accelKey is the critical setting

224 = Cmd (default) 17 = Ctrl 18 = option/alt

(B) Access keys for user interface hotkeys (usually underlined) and in-page navigation hotkeys (e.g., underlined letter in labels for form controls)

ui.key.chromeAccess - user interface (e.g., menu and dialog box underlined hotkeys) - you may need to change this to avoid conflicts with the change under (A), for example, you could swap this over to Cmd

1 = Shift [DO NOT SELECT!] 2 = Ctrl (default) 4 = option/alt 8 = Cmd

You can add those numbers together to specify combinations: 3 = Ctrl+Shift 5 = Shift+option/alt 6 = Ctrl+option/alt 7 = Ctrl+Shift+option/alt 9 = Shift+Cmd etc. 0 = disable this feature

ui.key.contentAccess - hotkeys inserted into the page by its author - you probably won't have a conflict here

6 = Ctrl+option/alt (default)

Same options as above

ui.key.generalAccessKey - suggest not changing this from the default value of -1 or the above will be overriden

(C) Menu bar access key - this is probably fine since it's the same as Windows

ui.key.menuAccessKey => 18 = option/alt

For comparison, the default values on Windows are:

ui.key.accelKey => 17 = Ctrl ui.key.chromeAccess => 4 = Alt ui.key.contentAccess => 5 = Shift+Alt ui.key.generalAccessKey - suggest not changing this from the default value of -1 ui.key.menuAccessKey => 18 = Alt