[hiliev]

Those are all necessary permissions since the plugin is basically building a completely new keyboard-driven chrome. It is not simply a skin that remaps a bunch of key bindings.

For example, to open a bookmark from the command line requires read access to your bookmarks in order for the plugin to present you with the list of bookmarks and eventually allow you to search or navigate it somehow. The same applies to opening something from your browsing history or resurrecting a closed tab. Tab switching requires access to the tabs. Clipboard access is for things like copying URLs or the selection in visual mode to the clipboard.

Vimium itself is open source on GitHub and you can see for yourself what it's doing with your data. The Firefox port is by one of the most active project contributors.