Hi Alex,
Thank you for prompt reply!
1. OK, good to know it has been on the table at least. Anyway, as a comment on to its popularity: It might be due to fact that adding music from your own collections is not that straightforward, specifically if you are not that tech-oriented person. Still, I guess also many non-tech persons have many own collections from past, and would love to have them at Spotify too. But since it is "hidden" feature, which needs some digging to find out how to do that, most of them just drops the issue thinking it is impossible or too hard to find how to do it. At least I would suppose there would be larger need for that. Maybe growing feature, as more and more "older" people also get used to Spotify.
Anyway, regarding that just to let you know if it would require some improvement: I did not find straight guidelines at Spotify -pages. But this is how I did it: Before Spotify part, one would need to rip the cd's (nice if this would also become part of Spotify destop-app). While relatively easy, it requires some technical knowledge. Then to add them to spotify: 1) install windows/mac/linux app 2) add new local files folder 3) create a playlist for those files 3) go to your library and find there the local files-icon 4) copy those files to new playlist. (now, regarding my point nr 2 - if one would repeat the process for the next cd, the old ones are here in the same list, without folder separation which makes it difficult to spot only the songs from the second cd, or another local folder. To make sense of it, before adding new one one needs to disable the old local folder at Settings, and then add the new one. Though when doing that, the old ones are not accessible anymore, so one needs to re-add that folder after all cd's are added..). 5) with mobile phone, make a local copy so you can listen to that playlist also on that mobile phone.
Now, adding to that last part: If one would do as regarding to original request: to replace songs with their matching equivalent from Spotify (to get metadata, graphics and save space on mobile etc), the download files locally -option loads not only your local files, but also the spotify-ones. While this is good for offline usage, it does not help to save space at mobile phone, as if one would like to download only those that does not exist in Spotify databases (that is, thos of your own local files that does not have equivalent at spotify).
Anyway, as it requires that many steps it could be just too hard for people to find instructions how to do that. But maybe it improves over time 🙂
2. Comments above, at step 4. Maybe the earlier description was unclear, but that part above maybe makes it more clear. The problem can be easily duplicated. Just try to add songs from 2 different cd's / folders, and then at My Library->Local Folders, try to make out which song belongs to which local folder (there, they area all in one long list without any separation possibility like folder etc)
3. The Firefox version was newest. Could relate to cache, but just as information in case someone responsible of the webform tests it.