User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:73.0) Gecko/20100101 Firefox/73.0

Steps to reproduce:

For the last several months, I've noticed intermittent failures sending & recieving emails from multiple devices and multiple IMAP/SMTP servers "Timeout error"

I found this guide on protonmail that fixes it: https://protonmail.com/support/knowledge-base/thunderbird-connection-server-timed-error/

Keep in mind I have a 300mbps connection and a 1gigabit connection. These time outs are way too aggressive.

Actual results:

Server times out in 100ms

Expected results:

tcptimeout config variable should default timeout to 2,000 ms (2 seconds)

Reporter, actually the tcptimeout you are referring to is in seconds. If there is no response after 100 seconds then something is probably wrong. However, since it is adjustable, it's OK to make it longer if you want to.
Note: Bug 1618455 does address an issue with the tcptimeout during imap append (happens when copying messages between accounts or saving a draft or saving to Sent folder after sending a message) where the tcptimeout is temporarily dropped down to 20 seconds. This may be related to what you are seeing (and the duplicate Wayne is hinting at).

->INVALID per comment 1. 100s (!) is pretty long