IMO yes; it's safe. Older, "weaker" encryption is used between the browser (IE or Chrome) and the so-called "front" server, and data is unencrypted between the "front" and "back" servers; but all this takes place within your own PC. No unencrypted or weakly-encrypted data ever leaves the PC. Thus, the connection between your PC and the Web server you're using will be as secure as the Web server is configured to make it.
It's conceivable that malware could be written to exploit ProxHTTPSProxy, but the number of folks using it is pretty tiny, so I doubt anyone would bother.