I noticed that when blocking CFR and unblocking via the about:newtab#devtools page, I was no longer able to clear impressions. This didn't use the be the case and makes it difficult to debug without throwing out the profile.

I'm not able to reproduce this. Turned CFR off via ASR debug page, turned it back on and impressions are back to 0.
Can you please double check?

STR is:

1. Open ASR devtools, click show a few times on a CFR. Impressions should increment.
2. Block the message. Unblock the message. Impressions says 0.
3. Restart the browser. Check ASR devtools again

Expected: Impressions should still be 0 for that message.
Actual: Impressions says the old number before blocking/unblocking.

Weirdly enough if I add more impressions after unblocking, those impressions persist (i.e. if I had 6, and then block/unblock, do another 2, then restart, indexedDB still has 2 impressions)

https://hg.mozilla.org/mozilla-central/rev/f98b2b564191