Tried building Firefox 39.0 from Mozilla provided sources as there are several
security issues in 38 branch.
See:
https://www.mozilla.org/en-US/security/known-vulnerabilities/firefox/#firefox39


First modified the Makefile and distinfo to version 39.0 with the calculated sha256 sum for the sources (after verifying the sha512 and sha1 sums from mozilla ftp site)


The actual problem is:
 # make install
===>  Patching for firefox-39.0,1
===>   Converting DOS text files to UNIX text files
===>  Applying FreeBSD patches for firefox-39.0,1
Ignoring previously applied (or reversed) patch.
1 out of 1 hunks ignored--saving rejects to browser/app/nsBrowserApp.cpp.rej
=> Patch patch-browser-app-nsBrowserApp.cpp failed to apply cleanly.
*** Error code 1

Stop.
make[1]: stopped in /usr/ports/www/firefox
*** Error code 1

Stop.
make: stopped in /usr/ports/www/firefox



Or by using portmaster a different :

# portmaster -i firefox

===>>> Currently installed version: firefox-38.0.6,1
===>>> Port directory: /usr/ports/www/firefox

===>>> Gathering distinfo list for installed ports

===>>> Launching 'make checksum' for www/firefox in background
===>>> Gathering dependency list for www/firefox from ports
===>>> Update libGL-9.1.7_4 to libGL-10.4.6? y/n [y] n

===>>> Initial dependency check complete for www/firefox


===>>> Starting build for www/firefox <<<===

===>>> All dependencies are up to date

===>  Cleaning for firefox-39.0,1
===>  Found saved configuration for firefox-36.0.4,1
===>   firefox-39.0,1 depends on file: /usr/local/sbin/pkg - found
===> Fetching all distfiles required by firefox-39.0,1 for building
===>  Extracting for firefox-39.0,1
=> SHA256 Checksum OK for firefox-39.0.source.tar.bz2.
===>  Patching for firefox-39.0,1
===>   Converting DOS text files to UNIX text files
===>  Applying FreeBSD patches for firefox-39.0,1
1 out of 1 hunks failed--saving rejects to
b2g/installer/package-manifest.in.rej
=> Patch patch-bug1013882 failed to apply cleanly.
=> Patch(es) patch-browser-app-nsBrowserApp.cpp applied cleanly.
*** Error code 1

Stop.
make[1]: stopped in /usr/ports/www/firefox
*** Error code 1

Stop.
make: stopped in /usr/ports/www/firefox

===>>> make build failed for www/firefox
===>>> Aborting update


===>>> You can restart from the point of failure with this command line:
       portmaster <flags> www/firefox 





# uname -a
FreeBSD <hostname> 10.2-RELEASE FreeBSD 10.2-RELEASE #1 b7e5018(stable/10)-dirty:
Fri Jul 10 12:51:25 EDT 2015

# ident /usr/ports/www/firefox/Makefile.org 
/usr/ports/www/firefox/Makefile.org:
     $FreeBSD: head/www/firefox/Makefile 389344 2015-06-13 11:08:22Z jbeich $

# ident /usr/ports/www/firefox/Makefile
/usr/ports/www/firefox/Makefile:
     $FreeBSD: head/www/firefox/Makefile 389344 2015-06-13 11:08:22Z jbeich $

Here is also the diffs of the Makefile and the distinfo.


diff Makefile Makefile.org 
5c5
< DISTVERSION=	39.0
---
> DISTVERSION=	38.0.6



# diff distinfo distinfo.org 
1,2c1,2
< SHA256 (firefox-39.0.source.tar.bz2) =
7e25014deb926df9220c3ab40ac6816c13a04c4cf7bd89b08fc1743307df1096
< SIZE (firefox-39.0.source.tar.bz2) = 182881881
---
> SHA256 (firefox-38.0.6.source.tar.bz2) =
379a83c45c606aa046bf543a95f806024089e8794ee3001efd1ba2f7067e2c4d
> SIZE (firefox-38.0.6.source.tar.bz2) = 180322743




Also just out of interest tried removing the problematic patch ( patch-browser-app-nsBrowserApp.cpp ), this just caused more problems with patches.


# make install
===>  Patching for firefox-39.0,1
===>   Converting DOS text files to UNIX text files
===>  Applying FreeBSD patches for firefox-39.0,1
Ignoring previously applied (or reversed) patch.
1 out of 1 hunks ignored--saving rejects to b2g/installer/Makefile.in.rej
1 out of 1 hunks failed--saving rejects to b2g/installer/package-manifest.in.rej
Ignoring previously applied (or reversed) patch.
1 out of 1 hunks ignored--saving rejects to browser/installer/package-manifest.in.rej
Ignoring previously applied (or reversed) patch.
1 out of 1 hunks ignored--saving rejects to mobile/android/installer/Makefile.in.rej
Ignoring previously applied (or reversed) patch.
1 out of 1 hunks ignored--saving rejects to mobile/android/installer/package-manifest.in.rej
=> Patch patch-bug1013882 failed to apply cleanly.
*** Error code 1

Stop.
make[1]: stopped in /usr/ports/www/firefox
*** Error code 1

Stop.