Lets look at what happens in troubleshooting mode. Your graphics are set to a default value which is in essence the setting "hardware acceleration" disabled. Addons are disabled. The user interface is largely returned to its default Those later two can be made more permanent by using the reset button that comes up when you start in Troubleshooting mode. I really don't think those are your issue, but please try them anyway, including going into settings and disabling hardware acceleration. Finally antivirus/security suites have these "things" they claim to offer like anti scam and anti spam tools which they inject into the Thunderbird running process instead of using the normal addon route. Just like the malware they claim to be defeating really. When this goes wrong, there is no way to block these insidious worms within Thunderbird, like disable an addon. But safe mode blocks all addons and it is the addon interface that the antivirus vendors exploit for their malware er tools. So go to your antivirus product and disable an toolbar or scan or spam tools (it used to be trend in particular but these days most of the products are owned by about two or three companies so the name on the package is somewhat irrelevant.