How to compare files on ftp-server and in a folder in Linux?
For folders i used Krusader and, with some success, WinSCP over WINE (and, with much more success, with WinSCP in Windows, in another machine, either real or virtual).
Small problems are, Krusader is going to have problems if files are 0 sized. Despite that, everything else work perfect.
In older distros or krusader versions, some dates are going to be shown incorrectly, making it believe local or remote copies are older than their cunterparts, due to time differences between time zones and local and server.
WinSCP is much more robust to deal with differences in time and other things, but it is slower and if path exceds the 256 characters lenght (a windows cronic issue) it is going to crash.
In another machine, you need to mount the Linux folder (via Samba or the VM folder share method) as a disk. That could also helps with the 256 characters path cronic illness on Windows apps.
I know the question is pretty old, but, it is one of the first results on google, so this is my 2 cents of contribution.
Not the ideal solution, but Total Commander @ wine will do the task.
Could not get meld or kompare to use ftp://...
However if the ftp server is not to big you could use wget --mirror and then do the diff locally, I know it's not the best solution... but it may work.
I have not tried this but there seem to be a possibility to mount the ftp server, using ftpfs or fuse or something like that (see Mounting ftp host to local directory on top of FUSE or HOWTO: mount ftpfs under Linux).
Then you could use the normal diff tools since now the ftp server behaves like a local dir.