You shouldn't need a plugin to view SVG in Firefox.

http://www.adobe.com/devnet/svg/adobe-svg-viewer-eol.html
Considering that Adobe SVG Viewer went E-O-L and was discontinued on Jan1, 2009, my guess is that Firefox doesn't even know that you have it installed it. Firefox no longer looks in the Program Files/Firefox/plugins/ folder for plugins as of early last year, which is where those older plugins were "dropped into" for many years. That folder isn't even in Firefox anymore.

Yes, it is. With older Firefox versions it could be improved by coping SVG Viewer *.dll files into Program Files/Firefox/plugins/ folder. But not with newer versions. Is there any way to put Firefox 25 working with Adobe SVG Viewer?

Yes, what is?
SVG support in Firefox has been improved a number times since 2009.

If you needed to place those DLL's in the Program Files/Firefox/plugins/ folder in the past, that should still work. AFAIK, all you need to do is to create the /plugins/ folder and drop the DLL files in there.

Would have been nice to get the full "picture" to begin with.

Also see: Windows Media or other plugins stopped working after Firefox update article.

Not sure what the use of the plugin is though.

There's no way to make Firefox 4 or above work with the Adobe plugin.

There used to be a switch to turn off native SVG but that switch was removed as the browser UI contains so much SVG that it stopped working entirely when native SVG was turned off.