I added lots of RSS feeds into Firefox bookmarks. But some of the new feeds are not shown because of a limitation. Is it possible to increase the number of viewed feeds? (For example, for xxxxxx.com it should show the last 300 feeds.)
I'm afraid, in the case of live bookmarks (which I assume you are referring to) Firefox already uses the "maximum" number of available feed items, but most feeds itself have a limitation on the number of items available in a feed (e.g. some show only the last 10 or so, sometimes less and sometimes more).
Since this is a limitation of the feeds itself and not a limitation in Firefox, it can not be fixed in Firefox.
If the problem is that Firefox can't handle the number of different RSS feeds you are adding, you may be able to overcome the issue by organizing your feeds into folders.
I have used Sage and some of its "offspring" as well as Brief. Both of these extensions allow one to organize feeds into folders.
Maybe a more advanced RSS addon for Firefox may solve the problem.
See this article for reviews : RSS in Firefox: A Complete Guide.
The article itself is from 2007, but the described addons have only improved since then.
You can retrieve a list that includes all current RSS reader addons from here.