"restore previous history" is grayed out intermittently.
"restore previous history" goes missing intermittently. sometimes after a restart/shut down, sometimes after a win7 update, sometimes after any program update that requires a restart. generally have 4-8 browsers open with 3-8 tabs each. believe it started about 3 firefox updates ago. have not found any recent related questions. so starting a new one. happens on my laptop too.
Check if this occurs with a single instance of FF running. Several copies running in parallel may lead to the contention situation: if the copy #m (or a tab in it, for that matter) is updating the history, the rest can not restore, until the said copy has finished updating.