I have a bunch of pinned tabs and also some normal tabs that I get restored whenever I open Firefox. I often immediately select a tab or open a new tab and start typing an URL when the UI becomes visible. A few seconds after that Nightly will always select the right most (so last) pinned tab, no matter where focus was. Probably whenever that tab loaded or similar. I'd expect nightly to not change the focused tab after starting.

I have verified this is unrelated to any extensions, since this also happens in safe mode.

The priority flag is not set for this bug.
:mikedeboer, could you have a look please?

For more information, please visit auto_nag documentation.

I think there is some additional information that may be of value here:

I've observed that this happens for one specific page, the todoist app. I wonder if they do something like call window.focus() and Firefox honors it by selecting the tab.