This is a pkgsrc
2015Q4 issue. If you browse the link for firefox packages that have compiled on the projects pkgsrc build server you will see that firefox43 is missing.
It never built.
The project is now building 2016Q1.
You can search the
pkgsrc-Bugs to see if it has been fixed and if so, what the fix was.
The other resource is
http://pkgsrc.se. If you search that site for firefox in
2015Q4, you will see that the pkg has not been updated. The maintainer has moved on the firefox45 in
current