Can't use some combinations of modifiers for the shortcuts

[FatOrangutan]

Prerequisites

• I verified that this is not a filter issue
  • Filter issues MUST be reported at filter issue tracker
• This is not a support issue or a question
  • Support issues and questions are handled at /r/uBlockOrigin
• I performed a cursory search of the issue tracker to avoid opening a duplicate issue
  • Your issue may already be reported.
• I tried to reproduce the issue when...
  • uBlock Origin is the only extension
  • uBlock Origin with default lists/settings
  • using a new, unmodified browser profile
• I am running the latest version of uBlock Origin
• I checked the documentation to understand that the issue I report is not a normal behavior

Description

#106 was fixed around the Firefox 61 time-frame, back when certain combinations of modifiers (e.g. ctrl+alt) were not allowed by Firefox. Starting with 63, however:

secondary modifier (optional). If supplied, this must be either "Shift" or (for Firefox ≥ 63) any one of "Ctrl", "Alt", "Command" and "MacCtrl", which has not been used as the main modifier.

Steps to Reproduce

1. Switch to the Shortcuts tab
2. Try setting something to say ctrl+alt+key, or alt+ctrl+key or shift+alt+key etc.

Expected behavior:

The typed combination to show up.

Actual behavior:

The combination is reduced to ctrl+key, alt+key, alt+key respectively.

Your environment

• uBlock Origin version: 1.17.5b3
• Browser Name and version: Nightly 65
• Operating System and version: Windows 10

[gorhill]

Starting with 63, however

I want uBO to still work fine with Firefox 60 ESR.

Beside, it looks like Firefox UI will support assigning shortcuts in the near future, so I won't do any more work uBO-side regarding this.