Condense some css rules.
Enforce thumbnail download sequencing.

Gecko tends to download all the thumbnails in DOM order, which defeats the
point of the thumbnail download sequence.

Thus, schedule thumbnails one-by-one while the main image is loading (which
improves download speed on slow connections), and schedule the rest normally as
soon as the main image is ready.
Improve thumbnail sizes.

Instead of using a simple 1:1 pixel ratio, choose a perfect
real-pixel/thumbnail-pixel ratio, so that thumbnails are always resized in
perfect powers of two, filling a proportional area of the screen.
Account for effective thumbnail list width when sizing the main image.