An obscure bug in Firefox 1.5.0.7 on both the Mac and the PC, causes saving changes to fail if a null byte (0x00) has been pasted into a tiddler. This can happen quite easily because there is a common bug in poorly behaved Windows applications where they include the C/C++ terminating null in the string body. (It's happened to me when copying from Windows applications running under Parallels on OS X).
The attached ZIP file contains a TiddlyWiki file that contains a null character in the tiddler "FireFox?". Loading it up in Firefox and then trying to save changes results in the file being corrupted in a characteristic way. Taking the same file and replacing the null character with an ordinary ASCII character allows it to save changes correctly.