Hi Please check and confirm the settings are correct - under accounts - Composition & Addressing in the left sidebar. Make sure Automatically Quote the original message when replying is checked, then in 'When quoting' choose whether to start your reply above or below as you prefer.

I did check that option before I posted this bug report, and it was checked. It was working just fine with the previous version of Thunderbird. I just checked it a second time, for both my accounts, and it is still checked. But when I press either Reply button, I get an empty composition input box.

Does it work properly in safe/troubleshoot mode (hold Shift when launching TB)?

Yes, it did work properly, so I'll try disabling each addon when I have time. But now the Reply button is doing nothing. Sigh. Software never seems to be written in a universal way, so making changes or adding extensions can always fail in strange ways. Sorry for the metacomment.

The 3 most likely causes of why things work in safe mode but not normal mode: - incompatible add-on or theme - broken layout (Reset toolbars & controls at safe mode prompt) - hardware acceleration (disable in Preferences/General)

I experimented with the addons to start with. I found that three of the addons were marked with yellow boxes saying "... is incompatible with Thunderbird 91.3.0". These were: "Search results Sort by Date...", "Show Address Only", and "Use BCC instead of C". Disabling just these three addons restored the Reply functionality of Thunderbird. My suggestion to the developers would be: instead of just showing these yellow boxes, disable the addons to prevent malfunctions.

There are some replacements for TB 91:

https://addons.thunderbird.net/en-US/thunderbird/addon/full-address-column/

https://addons.thunderbird.net/en-US/thunderbird/addon/limit-non-bcc-recipients/

Developers seldom read this forum. Submit requests for enhancements to Bugzilla.

Thank you for the info. When I searched for a way to submit a bug report, I was directed here. There was no option for Bugzilla.