I am glad this extension exists, because I use deepl on daily basis, and this extension boosts my efficiency. Does right what it says.

Even though FF says "This is not a Recommended Extension", its code is actually very transparent and not malicious. Check by yourself on windows: C:\Users\your_user\AppData\Roaming\Mozilla\Firefox\Profiles\your_profile\extensions\{c8f79b34-c3ff-4ce4-bdf4-eefa15c87f98}.xpi