I am using Thunderbird 102.7 on Windows 10. I can not replicate your experience.

Some things that may impact; Having an anti virus solution that scans files on access. This is well known to cause major issues in Thunderbird. Having a disk file system that is not sane or logical. Files that are regularly updated are also those most prone to the file system messing up storage around where they are and more importantly how big they are. A lengthy delay may mean that somehow the file has been allocated a huge amount of space and is taking an aeon to load. Check the file in the windows file explorer for size. With Thunderbird closed open the filterrules.dat file in windows notepad and see how long it takes.

Note use the more troubleshooting information on the tools menu to access the show button against the profile. This is the guaranteed way to get to where Thunderbird has it's [profile, not where you or I might think it is.

Each account has files either in a mail or IMAP mail fo9lder in the profile. Drilling down to the appropriate account folder (described as the account directory in account settings) there will be a filterrules.dat file for each account.

Note further. Local folders has the filters for all POP accounts that use the single inbox of local folders instead of an account inbox in the folder pane.

Hi Matt,

Sorry for the delay in getting back to you.

I disabled MalwareBytes Antimalware and then tried to update my message filters. I was able to add, edit, and run filters.

You were correct the an AV was scanning files on access and that appears to be the cause of my problems.

Looks like I'll have to remember to disable MalwareBytes every time I need to update my filters.

Thanks you for your help!

Ed