I've been a long-term user of the "Show my windows and tabs from last time" startup mode but recently it's been letting me down. I almost always have one or two pinned tabs open (an email tab and an add-on called Zotero) and up to a dozen normal tabs, but since 33.0 (fedora linux) the pinned tabs almost never come back when starting a new session.

Furthermore, today several of my regular tabs failed to appear too. This is very frustrating as I can't remember what they were!