Here is the STR from Mark:

• Have the browser toolbox previously open on the debugger tab, with enough files opened that you'll be in overflow (don't know if its required or not).
• Close the toolbox, restart Firefox.
• Open the browser toolbox from the tools menu (I'm on Mac).

=> Notice that the toolbox window isn't fully focussed, you can click on the source area, but not the toolbar/source code tabs.

• Click to main Firefox window and back to toolbox.
• Wait for about 10 seconds

=> The source code tabs magically disappear.

This seems to relate to the following code, removing the sourceId of all tabs when a target is destroyed:
https://searchfox.org/mozilla-central/rev/ec3889f74d6b5695833280f4370ca0e9ba59a3e4/devtools/client/debugger/src/reducers/tabs.js#127-129
Introduced in bug 1728587.

Set release status flags based on info from the regressing bug 1728587

:bomsy, since you are the author of the regressor, bug 1728587, could you take a look?
For more information, please visit auto_nag documentation.

persistTabs should only be used when we want to clear the tabs list
and have the persisted tab to be re-created (made visible, with a sourceId) when a new source matching the same url is created.

Also cleanup a few things and avoid returning new state objects unless we really changed something.

https://hg.mozilla.org/mozilla-central/rev/95732dfc8939

Please nominate this for ESR102 approval when you get a chance.

It sounds fine riding the train as this mostly only impact the browser toolbox.