That did the job. Although the sqlite interface is awfull (no response on what is going on, and I had to use putty again) the index was made and now the browse-failed message is gone.
I think adding this to firefly as standard functionality will be a good thing. I mean that the songs-database is automatically set up with indexes on the browsing columns (ie artist, album, genre). With a full scan I think the best thing to do is: first drop the index and then re-create it again. Then the actually full scan. With the normal-scan I think you can leave the index as it is, it will be updated because of the updated mp3-directory content.
I will try if the browse functionality is also working if I put the MP3-directories together. I will let you know if that’s working too.