Firewalls are not at all foolproof. So, no, that's not really adequate protection. Besides, if you use a vulnerable tool to communicate through the firewall... I'm sure you get the point.
Some good news for you: Uelsk8s built 62 packages for VL 5.8, a final set of updates. Some deal with security and some don't.
The bad news: these are the last set of updates for VL 5.8. When 6.0 releases there will be a formal End of Life announcement for 5.8. The repo will go into the "attic" with VL 5.1 and prior.
One upgrade every 2+ years is reasonable, IMHO.