When I start Firefox, one tab overwrites another.

Many times when I open Firefox, one of my open tabs loads as something that is or has been in a different tab. (Usually it's something that's currently open, but occasionally it's something that's been open in the last couple of days.) The tab history belongs to the new tab; I can only get back to the old tab by typing in the URL. Most often, the overwritten tab is the one furthest to the left, but not always. I originally though this was either an app tab or Gmail thing but since I've stopped using an app tab and leaving Gmail open, other tabs have taken the role of the new tab. Occasionally a tab also gets overwritten when I refresh it in the middle of a session. This problem only seems to occur with my main tab group; I have never seen it happen with other tab groups. In addition, I also use Firefox on Windows, and have never had the same problem there. I first noticed this problem with Firefox 6, if I recall correctly; it was whenever app tabs were first introduced. I have tried disabling all my plugins one at a time and it didn't help.