It is possible to trigger exception in the debugger for unknown threads. This is usually a symptom of reloading really quickly. I believe it is harmless because redux actions would have failed anyways with a context error, but it might be worth looking into having the errors occur sooner or have the messages be more clear.

JavaScript error: resource://devtools/client/debugger/src/client/firefox/commands.js, line 82: Error: Unknown thread front: UnknownThread

I see this error when opening the devtools with a watch expression in place. The error happens because the watch expression is evaluated when the expressions panel is mounted, at which point we may not have connected with the target tab's main thread yet. The evaluation fails, and the watch expression values stays as (unavailable) instead of the value presently on the page. It seems more appropriate to evaluate the expressions after connecting with the target thread (similar to what we do when pausing/resuming), which allows the expression to consistently update with the correct value as the debugger loads. There are certainly other reasons this error could show up, but other instances probably also indicate real problems with the debugger.

https://hg.mozilla.org/mozilla-central/rev/10eb7fee625f