Alt+1 through Alt+9 is all Firefox gives you for jumping to a tab by position, and it stops at nine. In a window with twenty-five tabs, most of them have no keyboard route at all: you cycle with Ctrl+Tab or you reach for the mouse. Tab Picker puts a letter on every one of them. Press Alt+A, press the letter, you are there.

One keystroke for the first twenty tabs, two beyond that.
Marks are positional and stable, so the tab in slot 3 is always "d". After a day you stop reading the strip and just press.
1-9 and 0 also reach the first ten tabs, so existing Alt+1-9 habit carries over.
Press / to search by title or host instead, with arrow keys and Enter.
Works on every tab in the window, including ones Firefox has unloaded to save memory, and including about: pages.

Where the marks appear: Firefox does not let any extension draw on the tab strip, so the marks are drawn on a mirror of it, at the very top of the page, directly under the real tab bar. By default each entry sits under the tab it stands for, at that tab's exact width, with its mark on a row of its own beneath it. In a crowded window it reads better than the real strip does, because the real one runs out of room around fifteen tabs and hides the rest behind scroll arrows, while the mirror shows every tab. On pages that refuse extension scripts, such as about:config or addons.mozilla.org, the same marks arrive in a toolbar panel instead, so the shortcut never simply does nothing. Permissions, plainly: it asks to access browser tabs, because a mark without a title or favicon next to it is useless. It does NOT ask for access to your data on websites. The strip is drawn only in the tab you are already looking at, under activeTab, which your own keypress grants. Every other tab is read as metadata and never touched. Nothing is collected, nothing is sent anywhere, no page content is ever read. Default shortcut is Alt+A, rebindable from Preferences with a recorder that validates the combination, warns about known Firefox collisions, and lets you test in place whether it is actually free. MIT licensed.