I've always wanted to get the old-timey Netscape "N" throbber working in Firefox, just for some of that retro feel. I found an old copy of the "N" throbber in my files and updated it for use with Firefox.
Here are some instructions for changing your throbber.
And here is an archive with the necessary files for that 90s feel. (Soul Asylum MP3s not included.)
The original throbber was 16 colors (EGA) and 30x30, but Firefox uses 16x16, 20x20, and 24x24 throbbers. Scaling the old dithered throbber looked awful, so I converted the GIF up to RGB, replaced the dithering with a solid color and made resized versions. However, the original GIF had an EGA palette, so converting the resized GIFs looked awful -- what I needed were more shades of the colors used in the image. With a limited palette, I could get several shades of each color within the 8-bit limit (DOOM used a similar trick). So, I stretched and blurred the RGB image and converted it back to 8-bit to make a reasonable indexed colormap (here are my source files). Then, I applied that map to my resized GIFs. Now you can party like its 1995!