Thanks for the review!

The reason for only two modifiers is that Firefox doesn't allow a third modifier key :(. You can see this on Firefox's built-in "Manage extension shortcuts" page - if you enter ctrl+alt+shift+ it'll display an error "Invalid Combination". Chrome has a similar oddity - it specifically forbids the use of both Ctrl and Alt in the same shortcut.

If you like you could use a shortcut like Alt+Shift+Left/Right for "move left/right" and Ctrl+Alt+Left/Right for "move to start/end" - so basically moving a finger to a different modifier key rather than having to move from cursor arrows to 0/9.

My original rationale for choosing '9' for moving the current tab to the end was to match the Firefox built-in shortcut of Ctrl+9 to go to the last tab. I selected '0' for moving to the first tab mainly because it sort of made sense and was very close to 9 on the number row.