Suggestion for Small Improvements
I just tested it and found it almost perfect !
Could it be possible to have the Floating Panel re-sizable and being able to move it ?
I'd really like also to have an option to make the panel appear in the bottom right side of the screen, this way it would not overlap text and would be not in your way when you translate often.
BTW: what is this option ?

Best Regards :blush:
panel is now resizable you can try the dev version here, https://github.com/schomery/dictionary/blob/master/builds/packed/firefox.xpi?raw=true
being able to move it ?
This is still not possible. Please open a bug report for it to discuss the possibility in a new thread.
I'd really like also to have an option to make the panel appear in the bottom right side of the screen
I am not sure how to do this. The height of panel depends on the content of the panel. This way panel needs to be resized each time.
BTW: what is this option ?
I must have forgot to remove the link from production version. This is my template for the options page