I can reproduce this exactly as described. The thing is: The browser deletes the cash completely on the first click, only it takes a while and the numbers shown in the preferences pane update too quickly, i.e. before the process is finished. Instead of clicking the button repeatedly you can also just hit cmd+R several times to see how quickly the cache is emptied. Or wait a few seconds (depending on cache size), then hit cmd+R and it will be zero.
Yes, I can reproduce it as well. It's not so much a bug as an assumption on Mozilla's part that disk writes are "free." I'm not really sure how to solve it other than just replacing the string with something like "Disk cache clearing in progress." or something, though that string doesn't exist in the locale that I've found.