I have an HTML signature with a web-based image. The image changes about once a month, but Thunderbird keeps using an old image. I believe it's being cached somewhere.
Any ideas on how to ensure the image is dynamically fetched every time?
How about generating a new filename each time, and dynamically updating your sig file as well? More work than just replacing the image, but less work possibly than changing the server header response.
On the server side:
Add an adequate Cache-Control header in the server's response for the image. E.g.
On the client side:
Set browser.cache.check_doc_frequency to 1 in Tools –> Options –> Advanced –> General -> Config Editor...
Either way will work to prevent Thunderbird from using the image without checking the cache freshness, but you probably want to use the server side way to make sure your recipients see your signature correctly.