If you use Firefox sync, you could just install the Mercury browser, set up access to your Firefox sync, and use that instead of Safari.
If you are trying to import bookmarks from Firefox, the implication is that you are using Firefox on another computer. Most of the instructions in this list refer to work-arounds from either a Mac or a Windows machine. If you are not using one or the other, you should probably specify what your situation is. I use a Linux desktop (barbaric, I know), so the Firefox sync with Mercury works for me.