How can I filter email to just show from a certain date forward?

you could use the date feature and select 'is after'

Check out the "Search Messages" feature (see menu item <Edit | Find | Search Messages> or hit Ctrl+Shif+F. Here you can create fairly complex searches, including date ranges. This can be a one-time search, or you can save the search as a "virtual folder" that will easily accessible and will update itself.

Here's an exhaustive article about the feature:

https://kb.mozillazine.org/Saved_Search