KeyboardNav от Reimar

This addon provides you with a set of customizable keyboard shortcuts that let you focus different parts of the web page or click a specific button.

Each shortcut has a CSS selector that, when pressed, will either be focused or clicked. If no elements matching the selector is found, nothing happens. If there are multiple, it'll go through them one by one as you keep pressing that shortcut.

The default shortcuts are:
CTRL + ALT + S: Focus the search bar
ALT + N: Focus the navigation bar
ALT + M: Focus the main content
ALT + A: Focus the sidebar
CTRL + ALT + H: Focus the page header
CTRL + ALT + F: Focus the page footer

Of course, these can be fully customized to your needs. You can also export/import them as a JSON file.