Usually when a script is written it is written not to run in conjunction with any other scripts that are already running, so once you introduce a new script there will be issues.
This year for scripts they (the authors) have taken a massive leap forward in terms of coding, with an end result of the unframing of Mafiawars not really needed to run the majority of them. But with this happening and all of them offering a simple import script option, these are the scripts that are conflicting with them. Not all scripts conflict! some get on quite nicely, however some scripts remove elements (Code) that other scripts depend on, if the script cannot find the element that another has removed, it causes a conflict and either stops!!! or doesnt work how the script is intended.
MW Addon (found here)
Spockholms (found here)
UnlockedMW (found here)
With the scripts that offer a script import service, All of these need Grease Monkey to be able to load the script.
What is Grease Monkey. Grease Monkey (or GM as its known for short) is a Mozilla Firefox extension that allows users to install scripts that make on-the-fly changes to HTML web page content on the DOMContentLoaded event, which happens immediately after it is loaded in the browser, when Greasemonkey scripts are persistent, the changes made to the web pages are executed every time the page is opened, making them effectively permanent for the user running the script (Unless disabled).
Greasemonkey can be used for adding new functions to web pages (for example, Loading any of the above 3 scripts), fixing rendering bugs, combining data from multiple webpages, and numerous other purposes (Source). There are also a few different variations of Greasemonkey but essentially they all do the same thing.
I use Firefox 5 (not 6,7,8 or whichever version they want to release every other week) I'm going to assume that you already have GM installed. If so you should notice the little monkey face somewhere on your browser (and i say somewhere because every version seems to put it somewhere else)
How to tell you have it enabled is by seeing it in a nonfaded detail compared to a faded pic
If you click on it, it will enable/disable it. If you click on the arrow by the side of it it will give you the next lot of options.
If you click on "Manage User Scripts..." you get the next screen
I highlighted the scripts in a red box, and their options on the right in a yellow box, Here you have control of which one or more you want running at the same time. The options are exactly what they say, but right now we are only interested in "Disable/Enable & Remove" you can see i have 3 scripts disabled and only one enabled, when you change these options any page with the script loaded on it will require a refresh for the option to take effect. If you find yourself no longer using one and want to delete it, then simply click remove.
If you are having issues with one of these scripts simply disable it, and refresh the tab, sometimes scripts cause issues because Zynga changes code and the scripts need to be manually updated to reflect the changes and the authors dont always have spare time to do so straight away