Almost every single airline, hotel, car rental, and other travel-related company that remotely interests me has my e-mail address for their mailing list. 99% of the time, I don't bother to read their e-mails. After about a month, I will have 800-1000 unread e-mails piled in my inbox.

I wonder if there is a Thunderbird add-on that can do the following..

* Every month, scan my inbox for keywords (whether it's subject or e-mail) of my choosing.
* Depending on the keyword and if the e-mail is more than x days old, it will automatically delete it. For example, weekly deals from an airline can be deleted if not read within 7 days, but mileage statement e-mails from that airline can stay for 14 days.

Tried searching but had no success.

I think you can set up a message filter that contains the rules you described. You can then leave it deactivated when not needed, then when necessary, activate it and let it do its work on your inbox. Afterwards deactivate it again.

After spending another 20 minutes scouring Google, that's pretty much the only solution I've found so far. I was hoping for something a little more automated.

I never noticed the "Age in Days" option in the filters. I set that to 3-28 days, depending on the type of e-mail, set it to Delete, then run it when it checks e-mails. Neat.