For search in Thunderbird see https://support.mozilla.org/kb/quick-filter-toolbar , and https://support.mozilla.org/kb/global-search

None of the info you supplied applies to my problem! In the dialog "customize message views" (ie -flagged, not in address books, recent mail, etc) from which one can edit any of them. I created one called "Lots of Ads" of which I have also sent a new screenshot with the 2 linked Dialogs.I have about 50 filters here There is NO way to search through these. Looks as though the search box is missing. Please re-look at my screenshot!

It would seem you are wanting to search your list of 'From contains' x, where you can check to see if you have already added a specific 'x' in order to eg: delete it or add it if not there. Easy to manually search if there is only a few, but with 50 plus it would take careful scanning.

You are talking about... 'Menu icon' > 'View' > 'Messages' > 'Customise' select a customised view and select 'Edit' I do not recall there ever being any means of performing a search on actual filter conditions within a filter and if located adjust the scroll bar to display it.

Also Message Filters 'Menu icon' >'Message Filters' > 'Message Filters' allows a search for a specific filter, but does not provide any means of searching for a specified condition set within a Filter itself.

I can see how searching to check for conditions/rules may be a useful aid if you utilise this feature as you do.

However, if you select that customised view, you could use a search as described by christ1 to see if you have any emails with a specified 'From' in the list displayed. If no results, then either the filter does not have the condition or condition needs modifying to pick it up or nothing has been received for that condition. But if you get a result then you know it is in the filter view as a condition/rule.