> use your browsers "save as" feature to save a copy
> Don't attempt to use the browser File/Save menu option to save changes (it doesn't work)
It looks like TiddlyWiki doesn't recommend that exact method anymore.
I use TiddlyDesktop and open with it the HTML file that I downloaded from the site. The desktop app takes care of saving the file when you finished editing a post and save it.
Other options do things like public hosting.
Personally, I just use the HTML file with the Timimi Firefox plugin installed for autosaving the current file.