I'm running Sandboxie v.5.31 beta on a new laptop with Firefox 67.0, Windows Home 64 bit.
My antivirus is Bitdefender Total Security 2019 and I also run Voodooshield.
When I start Firefox there is a message "SBIE2335 Initialization failed for process bdwtxcr.exe [33 / 0]" and the browser then starts normally.
EDIT: This could be unrelated, but I couldn't post this message using Firefox and I used Edge instead.