Add-on Converter for SeaMonkey - this is an online tool for converting Firefox and Thunderbird extensions to SeaMonkey.
Due to many similarities between these products it is often very easy to adapt an extension to work in SeaMonkey - the first step is to enable installation in SeaMonkey by adding an entry to install.rdf. The second step is to add SeaMonkey-specific overlays in chrome.manifest - sometimes their chrome:// URL's are different from those in Firefox but their equivalents exist and are pretty straightforward to detect. Then there are a few other differences like different ID names, different calls to certain services (like SMILE instead of FUEL/STEEL), etc. These are the things that this automatic converter tries to replace.
Of course, due to different levels of complexity of various extensions it is not always possible to port them so easily to SeaMonkey so the conversion will not always be possible. However, there is quite a number of extensions that appear to convert without problems - in the discussion thread about converter you will find a list of extensions that work and don't work along with users reporting more.
If you are developing a Firefox or Thunderbird extension you are encouraged to try out the converter - it may happen that your extension will work in SeaMonkey as well, in which case you might make your extension to support SeaMonkey out of the box with minimal effort on your part. The converter will allow you to see diffs of changed files so you can immediately see what changes would be required.