Masjlomcha! :-)
Toda for the rating.
On the FRs:
- Activate manually: That's on my Todo list. But haven't made up my mind how to implement. There are several ways and haven't figured the best yet. A toolbar button would be the most intensive work, but maybe a small dropdown menu in the window? Hmm
You know you can already fiddle a bit with the domain options, right?
- Colors: Not interested & too difficult for now. But there are addons that allows you to style any web page (and thus my in-page addon) any way you want. Like the addon "Stylish". Even if I can introduce customized styling, I think Firefox security wouldn't allow this unless with too much programming on my end.
- Window size: You can. There should be a corner that you can use. To remember that choice however would be very difficult because of already other schemes to size the window depending on many options. Prepared to discuss it though.

To further discuss any of the features, I propose to use the support site for that (e.g. create Issue, labeled "enhancement")

UPDATE 2018-04-24
I spent days on implementing features that might cross your desire to manually control (via a toolbar button and options). Have a look at the new version 1.0b. And don't forget the minimize button, although not by default.
Support site: Yes, best one for each request that remains.
P.S. We don't get notified of review edits. Just noticed because I wanted to reply here too (and couldn't).