great productivity booster!

This is a great Addon. So far so good.
Feedback
In any YouTube video, I can't find horizontal scroll bar. Also there are two huge horizontal black bar. (What I'm trying to say is that the Addon is unable to adjust according to the video's aspect ratio.
Again, great Addon.

very useful extension :)

Love this feature, thanks! Am so glad I discovered it. A bit concerned that Firefox Test Pilot seems to have ended (before I even discovered it). Hope that this excellent add-on does not disappear.

Great tool for developers! You can see the desktop and mobile versions of a page in one window.

An excellent tool. So useful across so many situations. One of the best is the ability to have my password manager available full time in the sidebar while updating logins. Or simply, anytime I want to keep an important site open and visible while doing other things. Should be a standard feature.

Just perfect

Just cannot use a browser without this. Amazing.

It's a fantastic tool, please improve it! For example loading bookmarks directly in side view or a bar on the left with bookmarks (icons) to open directly in side view.