Try viewing the search results as a list. There is a link for that at the top of the default results page

Thanks, but unfortunately this does not solve the problem, because: A client of mine writes several e-mails a day with another client of his. Then at some point he looks for a specific mail from him. He then remembers, to the nearest month, when this mail arrived, but that's all. That means he has to read aaaalllllll the mails briefly. He needs the preview of the mails in the search results. But with the list he would have to click on every single mail again and that doesn't help him.

Actually there can be only one solution: Increase the number of search results from 10. But how/where?

The solution you seek may not exist, but don't take my word for it. Someone else may have an idea of what you could do. The answer here https://support.mozilla.org/de/questions/1247914 explains why the results are presented in that manner. What I don't quite understand, is how it is such a bother to simply click the more button. Let's assume the message being searched for is the 1000th hit. Do you think it would be easier to find it if the results were ALL presented in one long page? Can't the results be refined further using terms (subjects) contained in the message body?

Well, don't tell me that... I am much more structured and orderly than my client of chaos... :-) The better use of the keyword seems to be useless for him. The stupid thing is that he does come from Outlook and it worked for him there with all the search results. Now he has a much better software with Thunderbird (*g*), but that's exactly what he can't do now. Hmph.

I'd hate to be you right now. I have had similar clients who I frankly do not miss! Sounds like this is too important for him, enough to send them back to Outlook. Let's hope someone else has a workable solution. I did my fair bit of searching and I'm simply not commiting any more time and effort to it. Sorry buddy.