Very good job, simple, efficient, close to perfect.
A few things I would suggest, by importance:
- add (customizable) keyboard shortcut
- keep the search bar open when going to another tab and then coming back to the one searched
- add option for case-sensitivity
- add option to keep the same set of searched words across tabs / pages when clicking on the icon
Possible bug: shouldn't the counter start at "1/..." and not "0/..."?
It looks like it doesn't work fully on the search result pages of linguee.com.