It shows a little information box if you hover over an -link which contains the target _blank
Just a little experiment for myself to test the firefox extension api and just idk :D Feel free to contribute
Weirdly the firefox team accepted my first attempt to publish this addon on their extensions marketplace
Here is that link: https://addons.mozilla.org/en-US/firefox/addon/show-if-link-is-target-_blank/
If you are searching for a serious addon i would recommend this one: https://addons.mozilla.org/en-US/firefox/addon/target-_blank-toggler/