Since there are so many keyboard layouts, operating systems and browsers, it's hard to find shortcuts that aren't clashing with any of the established ones and are still easy to use. IMHO Tab, and keyboard shortcuts for Task switcher on Windows, Spotlight on Mac and focusing the search box in Firefox all fall into this group, some more than other.
I believe the right solution here is to first find a more or less unique shortcut, even if it's a bit less handy, and then fix bug 1355835.
I've found an option with the Ctrl + Shift combination, which we use with several other shortcuts:
- Copy next helper: Ctrl + Shift + ↓
- Copy previous helper: Ctrl + Shift + ↑
We could also make sure that Alt is not pressed to avoid the clash with GNOME: