Approximately half of OOM small crashes I looked at have stack frame with symbols as
nss3.dll _PR_NativeRunThread(void*) nsprpub/pr/src/threads/combined/pruthr.c:399

Crash report: https://crash-stats.mozilla.org/report/index/c9165ede-471d-4969-bf0b-f869b0200822

Top 10 frames of crashing thread:

0 xul.dll xul.dll@0x2ad91e 
1 xul.dll xul.dll@0x2a2c7e 
2 xul.dll xul.dll@0xa70cb9 
3 xul.dll xul.dll@0xa70786 
4 xul.dll xul.dll@0xa70387 
5 xul.dll xul.dll@0xa712d1 
6 xul.dll xul.dll@0x3369f9 
7 xul.dll xul.dll@0x339545 
8 xul.dll xul.dll@0x7bf3f6 
9 xul.dll xul.dll@0x795c0c 

Half of them have uptime of 8-10 minutes. Other examples bp-d83bf6a9-bfb0-4769-bbbb-1aac20200822 and bp-63f6954f-4d84-46e1-9163-314720200823

bp-a98ecc51-0025-4c57-b304-1ae440200823 is 5 hours.

https://hg.mozilla.org/releases/mozilla-esr78/file/tip/nsprpub/pr/src/threads/combined/pruthr.c#l399
Looks like useless information. I think all that says is it crashed while it was doing whatever happened during that thread execution - but we don't know what that was.

Not sure if this helps, but I got a dialogue. I'll paste it here:

A script on this page may be busy, or it may have stopped responding. You can stop the script now, or you can continue to see if the script will complete.

Script: chrome://messenger/content/msgMail3PaneWindow.js:2041

I then checked a couple things in error console:
09:27:15.249
[Exception... "Component returned failure code: 0x80520001 (NS_ERROR_FILE_UNRECOGNIZED_PATH) [nsIXPCComponents_Utils.readUTF8URI]" nsresult: "0x80520001 (NS_ERROR_FILE_UNRECOGNIZED_PATH)" location: "JS frame :: resource://gre/modules/L10nRegistry.jsm :: L10nRegistry.loadSync :: line 658" data: no] L10nRegistry.jsm:658:19
loadSync resource://gre/modules/L10nRegistry.jsm:658
fetchFile resource://gre/modules/L10nRegistry.jsm:573
generateResourceSetSync resource://gre/modules/L10nRegistry.jsm:478
map self-hosted:240
generateResourceSetSync resource://gre/modules/L10nRegistry.jsm:473
generateResourceSetsForLocaleSync resource://gre/modules/L10nRegistry.jsm:415
InterpretGeneratorResume self-hosted:1151
next self-hosted:1099
generateBundlesSync resource://gre/modules/L10nRegistry.jsm:177
InterpretGeneratorResume self-hosted:1151
next self-hosted:1099
touchNext resource://gre/modules/Localization.jsm:167
regenerateBundles resource://gre/modules/Localization.jsm:552
activate resource://gre/modules/Localization.jsm:243
<anonymous> chrome://messenger/content/mailWidgets.js:37
<anonymous> chrome://messenger/content/customElements.js:34
<anonymous> chrome://messenger/content/customElements.js:37
observe resource://gre/modules/MailGlue.jsm:198

09:27:16.030
Error while loading 'jar:file:///C:/Program%20Files%20(x86)/Mozilla%20Thunderbird/omni.ja!/chrome/messenger/search-extensions/twitter/manifest.json' (NS_ERROR_FILE_NOT_FOUND) Extension.jsm:570
readJSON resource://gre/modules/Extension.jsm:570
onStopRequest resource://gre/modules/NetUtil.jsm:128
readFile chrome://openpgp/content/modules/files.jsm:214
retrieveOpenPGPPassword chrome://openpgp/content/modules/masterpass.jsm:104
haveMasterPassword chrome://openpgp/content/modules/masterpass.jsm:56
init chrome://openpgp/content/BondOpenPGP.jsm:101
<anonymous> chrome://openpgp/content/BondOpenPGP.jsm:200
<anonymous> chrome://messenger/content/msgMail3PaneWindow.js:67
09:27:16.252
[Exception... "Component returned failure code: 0x80520001 (NS_ERROR_FILE_UNRECOGNIZED_PATH) [nsIXPCComponents_Utils.readUTF8URI]" nsresult: "0x80520001 (NS_ERROR_FILE_UNRECOGNIZED_PATH)" location: "JS frame :: resource://gre/modules/L10nRegistry.jsm :: L10nRegistry.loadSync :: line 658" data: no] L10nRegistry.jsm:658:19
loadSync resource://gre/modules/L10nRegistry.jsm:658
fetchFile resource://gre/modules/L10nRegistry.jsm:573
generateResourceSetSync resource://gre/modules/L10nRegistry.jsm:478
map self-hosted:240
generateResourceSetSync resource://gre/modules/L10nRegistry.jsm:473
generateResourceSetsForLocaleSync resource://gre/modules/L10nRegistry.jsm:415
InterpretGeneratorResume self-hosted:1151
next self-hosted:1099
generateBundlesSync resource://gre/modules/L10nRegistry.jsm:177
InterpretGeneratorResume self-hosted:1151
next self-hosted:1099
touchNext resource://gre/modules/Localization.jsm:167
regenerateBundles resource://gre/modules/Localization.jsm:552
activate resource://gre/modules/Localization.jsm:243
<anonymous> chrome://openpgp/content/modules/trust.jsm:11
<anonymous> chrome://openpgp/content/modules/keyRing.jsm:17
<anonymous> chrome://openpgp/content/modules/windows.jsm:19
loader chrome://openpgp/content/modules/lazy.jsm:20
getService chrome://openpgp/content/modules/core.jsm:426
getService chrome://openpgp/content/modules/core.jsm:183
init chrome://openpgp/content/BondOpenPGP.jsm:181
<anonymous> chrome://openpgp/content/BondOpenPGP.jsm:200
<anonymous> chrome://messenger/content/msgMail3PaneWindow.js:67

Let me know if there is more I can help with.

(In reply to Worcester12345 from comment #5)

Let me know if there is more I can help with.

Please post a recent crash ID with this signature, and include which AV software(s) you are using on this computer.

Nothing more recent that I know of.

(What do you mean "with this signature"?)

(In reply to Worcester12345 from comment #7)

Nothing more recent that I know of.

(What do you mean "with this signature"?)

"OOM | small" is the signature for this bug

Thunderbird crash rate is down about 20% compared to 6 months ago. Still, it is #3 crash for 78.10.1.

(In reply to Wayne Mery (:wsmwk) from comment #9)

Still, it is #3 crash for 78.10.1.

OOM | small is #1 for version 91 in Oct/Nov 2021 ... well, tied with #2 nsXPCWrappedJS::Release, which we think should be greatly helped by Bug 1742991 - crash nsXPCWrappedJS::AddRef

OOM | small via nss3 is about 10% of those crashes

Thunderbird crash rate has dropped at least 10% in the past 6 months.

(In reply to Wayne Mery (:wsmwk) from comment #11)

Thunderbird crash rate has dropped at least 10% in the past 6 months.

Does Thunderbird user rate correlate to that?