Actually, if you have (or had) the .RPM version installed, then you would have installed the package via your package manager of choice. You can kind of think of this means of installing the extension as a kind of system policy; it installs a single copy of the extension to the main Firefox application folder and enforces the use of AdBlock on all users of a system.
If, on the otherhand, you install the .xpi version of AdBlock Plus as an extension from within Firefox then it's a user specific thing, and this is the normal way of handling extensions in Firefox. It's installed to a directory within the user's home directory and the user is free to remove it, but every user would need to install the extension separately.
Which approach you use is a matter of personal preference and requirements; the former is much more useful for system admins of multi-user installs, but for all others the latter is probably a simpler choice.
Andy