From checking that BugZilla thread, this issue affects Firefox ESR, the extended support release for organisations. The example David points to is another Discourse based forum.
To test, I installed Firefox ESR (60.7.1esr 64-bit) and as I write this reply the cursor is missing. The workaround they mention (minimise and restore) does indeed trigger the cursor to appear.
I will reply to that BugZilla thread to mention this.