I just stumbled upon two extensions for Firefox that each replace the Ctrl-Tab functionality in Firefox. Currently when you press Ctrl-Tab Firefox will shuffle through your tabs going to the most currently opened tabs first. It is kind of like the Alt-Tab in Windows except this is for all of your different Firefox tabs.
I never used the Ctrl-Tab feature in Firefox until now. This Ctrl-Tab Preview extension is absolutely amazing (pictured above). It will shuffle through your open tabs displaying a small preview for each one. This becomes extremely useful to me when I have lots of windows open.
There is also a similar extension called LastTab that will show you a preview of each window in a different way. It creates a popup window that has a thumbnail preview on the left side and a list of the open tabs on the right side. I prefer the first extension but this one may be more what you are looking for.
If you want to see thumbnails of all your windows at the same time then you are looking for the Viamatic foXpose extension. It will scale the thumbnails onto one page, so if you have just 4 windows open then your thumbnails will be much larger compared to having 16 windows open. I also like this extension but I found myself not using it very much. For right now I am going to stick with the Ctrl-Tab Preview extension.