This could be improved by having a “sticky” menu in which you right-click and scroll to bring up the menu, and can then click directly on the tab you want to switch to. You might also be able to satisfy your readers who want to “group” tabs by having a hierarchical menu pop up, but at that point you have to waste energy naming tab groups. Group your tabs by firefox window if you are going to have that many open.

I assume you mean the thumbnails should show up larger than they do in Acrobat than the default, so maybe one per row, right?

If I were to adopt your idea, I would actually get rid of the tab bar entirely, simply having a scroll bar of tabs to select, with their titles hovering over the thumbnails. This is because I always prefer to eliminate redundant mechanisms: while one mechanism may be sub-optimal compared to others in some use cases, not forcing the user to choose between mechanisms makes up for the deficiency.

Even modified, I still prefer my way, mostly because, for me, the most important feature is that the tab thumbnails be very large, preferably nearly as big as their full size when actually selected. In my experience, I often have many tabs open that look too similar as small thumbnails to be discernable from one another, so I need thumbnails big enough to read the page’s text to choose the correct tab.

By having large previews appear instantly upon mousing over a tab, it becomes cheap to browse tabs by visual recognition.