I'm not aware of any limit. Why do you ask?

because when i try to imports my Favorites from another browser it only imports half of my folders retry three times same thing happen

Aha. I don't know why that happens, but there is a workaround.

When the import is partial or incomplete, you can work around that by manually exporting from IE to an HTML file, then importing that file into Firefox.

Please see the following thread for a discussion of the process: Only 1/3rd of my bookmarks imported from IE. How do I move the rest?

Does that work?