Just to clarify, this is happening in the middle of sentences, and even trying to correct it takes multiple attempts as it just keeps trying to force this change on the random word it's chosen as it's victim. Sometimes even to more than one word in the sentence as well.

This behavior continues for the duration of composing a message

After startup, and after seeing the problem, do ctrl+shift+J to bring up error console What errors do you see there?

After startup:

1663510953717 addons.xpi WARN Checking /Applications/Thunderbird.app/Contents/Resources/distribution/extensions for addons Successfully loaded OpenPGP library librnp.dylib version 0.16+git20220124.f06439f7.MZLA from /Applications/Thunderbird.app/Contents/MacOS/librnp.dylib RNPLib.jsm:100:15 Found 0 public keys and 0 secret keys (0 protected, 0 unprotected) RNPLib.jsm:301:15 Exception { name: "NS_ERROR_FAILURE", message: "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsISecretDecoderRing.decryptString]", result: 2147500037, filename: "chrome://openpgp/content/modules/masterpass.jsm", lineNumber: 230, columnNumber: 0, data: null, stack: "_readPasswordFromFile@chrome://openpgp/content/modules/masterpass.jsm:230:16\n", location: XPCWrappedNative_NoHelper } RNP.jsm:47:15 services.settings: Failed to load last_modified.json: TypeError: NetworkError when attempting to fetch resource. Utils.jsm:330 Trying to load /Applications/Thunderbird.app/Contents/MacOS/libotr.dylib OTRLib.jsm:64:11 Successfully loaded OTR library /Applications/Thunderbird.app/Contents/MacOS/libotr.dylib OTRLib.jsm:72:13 tb.account.size_on_disk - Trying to set an unsigned scalar to a negative number. 2 tb.account.size_on_disk - Attempted to set the scalar to an incompatible value. 2 about:preferences : Unable to run script because scripts are blocked internally.

Immediately after problem manifests:

1663510953717 addons.xpi WARN Checking /Applications/Thunderbird.app/Contents/Resources/distribution/extensions for addons Successfully loaded OpenPGP library librnp.dylib version 0.16+git20220124.f06439f7.MZLA from /Applications/Thunderbird.app/Contents/MacOS/librnp.dylib RNPLib.jsm:100:15 Found 0 public keys and 0 secret keys (0 protected, 0 unprotected) RNPLib.jsm:301:15 Exception { name: "NS_ERROR_FAILURE", message: "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsISecretDecoderRing.decryptString]", result: 2147500037, filename: "chrome://openpgp/content/modules/masterpass.jsm", lineNumber: 230, columnNumber: 0, data: null, stack: "_readPasswordFromFile@chrome://openpgp/content/modules/masterpass.jsm:230:16\n", location: XPCWrappedNative_NoHelper } RNP.jsm:47:15 services.settings: Failed to load last_modified.json: TypeError: NetworkError when attempting to fetch resource. Utils.jsm:330 Trying to load /Applications/Thunderbird.app/Contents/MacOS/libotr.dylib OTRLib.jsm:64:11 Successfully loaded OTR library /Applications/Thunderbird.app/Contents/MacOS/libotr.dylib OTRLib.jsm:72:13 tb.account.size_on_disk - Trying to set an unsigned scalar to a negative number. 2 tb.account.size_on_disk - Attempted to set the scalar to an incompatible value. 2 about:preferences : Unable to run script because scripts are blocked internally. This page is in Quirks Mode. Page layout may be impacted. For Standards Mode use “”. blank This page is in Quirks Mode. Page layout may be impacted. For Standards Mode use “”. MsgComposeCommands.js:10508:14 uncaught exception: Unable to load script: moz-extension://e71fc263-1109-46d7-8586-294d21dfec41/fuzzysearch.js uncaught exception: Object ReferenceError: FuzzySearch is not defined textinsert.js:54:5 suggs <empty string> 4 textinsert.js:478:15

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MacOS 11.6.8 Thunderbird 102.2.2 When composing messages I have 2 problems that happen at random on a consistent basis. 1)Thunderbird randomly capitalizes the first letter of a word and adds a space between the first letter and the rest immediately after hitting the spacebar. Example: "problem" becomes "P roblem"

Thunderbird has no inherent auto correct function. Do you use an addon for this?

2)If I then hit the backspace key, it will type the word "backspace" most of the time, but not always.

This sounds like it may be simply another manifestation of 1.

I believe its either a setting somewhere or a corrupted file but I'm open to any suggestions. My hunch about this is because I had this problem on a windows machine that we migrated Thunderbird's files over from. We didn't think it was Thunderbird's fault at the time because after another unsolicited Windows 10 update the machine was doing all kinds of weird bonkers things that finally made us kick it to the curb metaphorically speaking.

I think the issue is some sort of external auto correct as Thunderbird does not have one of it's own. So either an addon, or something provided by the operating system.

Idk why such a simple idea eluded smh. Only have 1 add-on that I know of. Quick text. It allows me to insert pre designated phrases or words. I have it setup for custom greetings and signatures. But I'm going to go check to make sure theres nothing else and basically disable whatever I find. Turn back on one by one and test each change. Thanks for the idea! I'll report back with findings.

• Pretty sure its not OS related since it started on windows 10 and then migrated to the MacOS. All the data and profile folders were copied into the mac manually and this hasn't appeared in any other application.

Turns out there WAS an extension I didn't know about. After disabling it, the problem seems to have disappeared. MANY THANKS!