Hooray, an actively maintained extension that allows a custom URL in the firefox sidebar AND lets you choose a custom title for the sidebar entry! This one definitely does deserve a better rating - it is meant to present a fixed URL in the sidebar/popup box, and it cannot cirumvent the restrictions Firefox applies in this case.

The only thing I miss is the ability to set a default zoom level for the sidebar (if Firefox allows that) :-)

Zu Jedem Topf gibt es EINEN DeckeL (oderETWAnicht?).
There is a lid for each pot (or not?).

A good idea somehow likes "Pined URL button" though, but whatever address I set it doesn't display anything, blank in sidebar view, and nothing in popover view - it's not popping over.

Windows 10 1803, Firefox Developer Edition 62.0b12 (64bit)

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9/30 Edited:
Thanks for your hard working, now it works fine in sidebar, however still no popping over..

I understand this addon is actually a proof of concept and not really a working one.

However, as a former user of extension "all in one sidebar" (no longer supported) and later "omnisidebar" (no longer supported either) I would be really happy to use this one for the following.

I hate the way the "downloads" window is implemented in Firefox. I usually download things and they slowly accumulate in the list of completed downloads. If I click the toolbar button, the outline popup is usually too small to let me see the oldest files in the list so I have to make the full window appear, which clutters my screen with an additional, unwanted window. It takes a lot of mouse clicking to finally access that one file that was sitting at the bottom of the list.

I would love to have my list of downloaded files appear on a side panel which could easily be shown or hidden. Either AIOS or omnisidebar mentionned above used to implement that in a satisfactory way. I almost managed to get that beahvior with your addon by putting "about:downloads" in the address box. However, would it be possible that instead of having the toolbar button of the extension show an outline of the panel content (which, in this case, does pretty much the same as the "downloads" toolbar button) it instead would actually open the side panel?

If you prefer (and if the above is actually possible), you could also give the user the opportunity to choose between the two with a checkbox in the addon configuration.

Best regards and thanks for reading.