Can't test this extension because it doesn't support local files and the only SVGs I need to view are stored locally. The hard top/left rendering of the FF built-in viewer is nasty and I'd be enthusiastic about CenterSVG if it worked with local files.
There are two things I would love to be done: 1) Ability to set background color as yours is too dark and black images are poorly visible 2) Extension works only with SVG files that are browsed online, when I open saved SVG it's not affected by extension. I collect some SVG's for the work and need to browse them. Using Firefox for that purpose will be great.
Otherwise it's a useful extension indeed. I don't understand why all browsers center bitmap images, but doesn't center SVGs.
I have this suggestion: as SVG can be infinitely resized, how about adding an option to enlarge fit an SVG image to current window size? Kind of the same way FF already does for raster images, but backwards – enlarging instead of shrinking.