Plain SVG is a file format of inkscape and other svg-editors, made for compatibility between svg drawing programs, to strip of all program related information that is of no use in other svg-editors and because of filesize. It is a human readable format, because the filecontent is saved as ascii-text.
Another option is the svgz file format which is a normal svg file packed with gnu-z. That shrinks the filesize, but produces a binary which is not human readable as it is.
It is a good idea to save your work files in svgz format, to save space on your harddrive. Inkscape and some other programs as Ksvg can read svgz files without a problem or timedelay.
FireFox 18.104.22.168 can not display svgz-files directly. It must be converted to svg first.