The archive contains a port of SeaMonkey as well as the therefore needed, but deprecated port of lang/python27; Note that lang/python27 is just a build dependency, and can be removed afterwards.

Updating from version 2.49.5 or earlier means "no way back" (or "use your backup") as the user directory isn't backward compatible.

For a faster build process you may want to check if the following packages are installed on your machine: cmake gmake gtk2 gtk3 llvm90 rust-cbindgen; But consider that pre-installing those dependencies will set them to a "non-automatic" state, while the build process would leave them with the "automatic" flag; So you've got to take care of getting rid of them again by your own – a simple pkg autoremove won't do that for you…

If you don't want an english GUI take a look at the available language packs.
To get the spellchecker up & running you have to install a dictionary addon that will be explicit filed under (the on the fly uprising topic) "Dictionaries" in about:addons – and not under "Extensions"; That circumstance belongs to a newer addon meachnisms SeaMonkeys big brother uses, so you may have to dive through older versions of the desired dictionary (SeaMonkey has so far no WebExtensions api support, so for addons legacy versions are needed). And don't forget to select your dictionary in the preferences and restart SeaMonkey…

This port needs ImageMagicks bindings for Perl. As they aren't enabled by default you also need to compile the port graphics/ImageMagick7; Just having the package installed won't work.

Backed port: At time of building this "ports extensions" this was the actual and official state of that port; It's just included because it will be removed from the ports tree soon. But as it's needed to build SeaMonkey: With this you're able to pull it temporarily in even if it's officially gone.