As thedead1440 alread said, read-only mode is not possible. But you can edit the system default bookmarks. That way, whenever you reset your browsing history from settings your edited bookmarks get restored.
But you can also alter the access counter to ensure your bookmarks stay on top. But think access counter isn't the only value that decides were your bookmarks get placed. I think the rank and the last access date are important, too.

Read from here on to see what's to do. You'll need a tool for editing the SQLite, such as the "SQLite Manager" Firefox addon.