please direct me to your user guide. I've looked all over and don't see anything close.

For how to use filters, have you checked this article? https://support.mozilla.org/en-US/kb/organize-your-messages-using-filters

I think filters apply to new messages.

Sorry for delay. I think at the time I was hoping to find a cron function. In the meantime ChatGPT is helping me a lot. Like today a filter I was setting up wouldn't work no matter what I tried. The selection was working (I knew because I tagged it to change color), but it wouldn't move to a folder no matter what. Turns out the folder was corrupt which ChatGPT lead me to verify by trying to drag and drop the message into it. I also was hoping to be able to do some if then logic without having to make a separate filter. Found out I can't.

Thanks for replying and directing me to the article. Much appreciated.

Message Filters can be set to run automatically on new incoming mail that arrives in 'Inbox'. Enabled filters run in the order listed. A Manual run using any selected filter and any selected folder can be also be done.