Does your filter have a long list of From is <email address>?

Yes it does

I would recommend making the filter act on From 'is in my address book', and create an address book with the relevant contacts (see picture here.

I have several filters that contain a lot of rules. For instance I play tennis and I have maybe 30-35 people on tennis teams I receive email from. The filter has a rule for each of their email addresses listed and then moves the emails to a single folder on my hard drive. This is how I primarily use Filters, I have over 150 filters which move email into their own folders on my hard drive.

Another example I have a filter that I call 'Delete' which contains every junk/unwanted email that Thunderbird doesn't catch. So I hit the plus sign to add a rule for the 'FROM' address of each junk email with an action set to 'Delete' the email.

In both of these cases after the rules get long enough, they won't fit on the screen in one window. This window should be scroll-able when the list is long.

For your suggestion I wouldn't want to add every junk/unwanted email to my address book but thanks for the suggestion. Chris

I'm not aware of any limit on the number of rules, but in the case of your tennis example, you could create an address book named Tennis, then copy contacts from Personal Address Book to Tennis (drag and drop while holding Ctrl), and have a single rule as described above, instead of 35.

Same for unwanted mail. That would work for annoying but non-spam senders, but is very unlikely to work for genuine spam, where the From address is seldom the actual sending address, or even a real address.

The number of rules in a filter can also sometimes be reduced by using 'contains' or 'ends/begins with' criteria instead of 'is'.