Why do I have to open my vault every time I launch Firefox 49.0.2

If I launch Firefox 49.0.2 and then unlock my vault to enable autofill, that works ok.

If I then close down Firefox and then launch it again, I have to unlock my vault again in order to enable autofill.

Why is this? In both Chrome and IE, once I have opened my vault I can then close and relaunch the browsers as many times as I like and the vault always remains open.

It's a pain having to keep reopening my vault every time I relaunch Firefox.

Does anyone have a way round this?