Description of problem:
After running for some time Firefox runs out of file handles. It then begins writing errors to the journal like this:

Apr 27 15:25:06 felis firefox.desktop[5422]: (firefox:5422): GStreamer-CRITICAL **: gst_poll_write_control: assertion 'set != NULL' failed
Apr 27 15:25:06 felis firefox.desktop[5422]: (firefox:5422): GStreamer-CRITICAL **: gst_poll_write_control: assertion 'set != NULL' failed
Apr 27 15:25:06 felis firefox.desktop[5422]: (firefox:5422): GStreamer-CRITICAL **: gst_poll_get_read_gpollfd: assertion 'set != NULL' failed

And

Apr 27 15:25:19 felis firefox.desktop[5422]: console.error:
Apr 27 15:25:19 felis firefox.desktop[5422]:   Could not write session state file
Apr 27 15:25:19 felis firefox.desktop[5422]:   Message: Unix error 24 during operation open on file /home/zlynx/.mozilla/firefox/1wiu6mtz.default/sessionstore-backups/recovery.js.tmp (Too many open files)

The command lsof -p [firefox PID] shows (this is an extract):

irefox 5422 zlynx   66u     unix 0xffff880461a18f00      0t0 1616813 type=STREAM
firefox 5422 zlynx   67u      REG               0,47    32768 4989899 /home/zlynx/.mozilla/firefox/1wiu6mtz.default/webappsstore.sqlite-shm
firefox 5422 zlynx   68r      REG               0,47 39944192    7863 /home/zlynx/.mozilla/firefox/1wiu6mtz.default/webappsstore.sqlite
firefox 5422 zlynx   69u      REG               0,47  1049376 4989898 /home/zlynx/.mozilla/firefox/1wiu6mtz.default/webappsstore.sqlite-wal
firefox 5422 zlynx   70u     unix 0xffff880461a4f440      0t0 1608517 type=STREAM
firefox 5422 zlynx   71r      REG               0,47 73400320    7742 /home/zlynx/.mozilla/firefox/1wiu6mtz.default/places.sqlite
firefox 5422 zlynx   72u      REG               0,47    32824 4989890 /home/zlynx/.mozilla/firefox/1wiu6mtz.default/places.sqlite-wal
firefox 5422 zlynx   73u     unix 0xffff880461a192c0      0t0 1616814 type=STREAM
firefox 5422 zlynx   74u     unix 0xffff880461a19680      0t0 1616815 type=STREAM
firefox 5422 zlynx   75u     unix 0xffff8801231f6180      0t0 1621086 type=STREAM
firefox 5422 zlynx   76u      REG               0,47    32768 4989986 /home/zlynx/.mozilla/firefox/1wiu6mtz.default/healthreport.sqlite-shm
firefox 5422 zlynx   77u     unix 0xffff880005e14b00      0t0 1616197 type=STREAM
firefox 5422 zlynx   78u     unix 0xffff88017e9f2580      0t0 1688923 type=STREAM
firefox 5422 zlynx   79u     unix 0xffff880005e15280      0t0 1616198 type=STREAM
firefox 5422 zlynx   80u     unix 0xffff880005e11a40      0t0 1616199 type=STREAM
firefox 5422 zlynx   81u     unix 0xffff880461a48780      0t0 1608518 type=STREAM
firefox 5422 zlynx   82u     IPv4            1480796      0t0     TCP felis:58344->208.85.149.37:https (ESTABLISHED)
firefox 5422 zlynx   83u     unix 0xffff880461a4f800      0t0 1608519 type=STREAM
firefox 5422 zlynx   84u     IPv4            1560653      0t0     TCP felis:56892->ec2-52-90-163-180.compute-1.amazonaws.com:https (ESTABLISHED)
firefox 5422 zlynx   85u     unix 0xffff880005e15640      0t0 1616200 type=STREAM
firefox 5422 zlynx   86u     unix 0xffff880461a19a40      0t0 1616816 type=STREAM
firefox 5422 zlynx   87u     unix 0xffff880461a48000      0t0 1608520 type=STREAM
firefox 5422 zlynx   88u     unix 0xffff8801231f5dc0      0t0 1621087 type=STREAM
firefox 5422 zlynx   89u     unix 0xffff880461a19e00      0t0 1616817 type=STREAM
firefox 5422 zlynx   90u     unix 0xffff8801c1891680      0t0 1615680 type=STREAM
firefox 5422 zlynx   91u     unix 0xffff880461a1a1c0      0t0 1616818 type=STREAM
firefox 5422 zlynx   92u     unix 0xffff880123162d00      0t0 1612389 type=STREAM
firefox 5422 zlynx   93u     unix 0xffff8801c1895dc0      0t0 1615681 type=STREAM
firefox 5422 zlynx   94u      REG               0,43   557056 1475993 /tmp/mozilla-temp-504519083 (deleted)
firefox 5422 zlynx   95u     unix 0xffff880005e130c0      0t0 1475994 type=STREAM
firefox 5422 zlynx   96u     unix 0xffff880461a4a1c0      0t0 1608671 type=STREAM
firefox 5422 zlynx   97u     unix 0xffff880005e12580      0t0 1475995 type=STREAM
firefox 5422 zlynx   98u     unix 0xffff880005e13fc0      0t0 1475996 type=STREAM
firefox 5422 zlynx   99u     unix 0xffff880461a4b0c0      0t0 1608672 type=STREAM
firefox 5422 zlynx  100u     unix 0xffff880005e14380      0t0 1475997 type=STREAM
firefox 5422 zlynx  101u     unix 0xffff880461a4bfc0      0t0 1608673 type=STREAM
firefox 5422 zlynx  102u     unix 0xffff880005e13840      0t0 1475998 type=STREAM
firefox 5422 zlynx  103u     unix 0xffff880005e13c00      0t0 1475999 type=STREAM
firefox 5422 zlynx  104u     unix 0xffff880005e13480      0t0 1476000 type=STREAM
firefox 5422 zlynx  105u     unix 0xffff880005e14740      0t0 1476001 type=STREAM
firefox 5422 zlynx  106u     unix 0xffff880461a4cb00      0t0 1608674 type=STREAM
firefox 5422 zlynx  107u     unix 0xffff880005e103c0      0t0 1476003 type=STREAM
firefox 5422 zlynx  108u     unix 0xffff880005e10780      0t0 1476004 type=STREAM
firefox 5422 zlynx  109u     unix 0xffff880005e10b40      0t0 1476005 type=STREAM
firefox 5422 zlynx  110u     unix 0xffff880461b58000      0t0 1475434 type=STREAM
firefox 5422 zlynx  111u     unix 0xffff880461b583c0      0t0 1475435 type=STREAM
firefox 5422 zlynx  112u     unix 0xffff880461a4b480      0t0 1608675 type=STREAM
firefox 5422 zlynx  113u     unix 0xffff880123164ec0      0t0 1612390 type=STREAM
firefox 5422 zlynx  114u     unix 0xffff880123162940      0t0 1612391 type=STREAM
firefox 5422 zlynx  115u     unix 0xffff880123163480      0t0 1612392 type=STREAM

And after that it is unix stream sockets until 1023u.

Extensions installed (although if this is an extension bug, it is still a BUG because all of these work in Windows):
ADB Helper
DownThemAll!
HTML5 Notifications
NoScript
PageZipper
User Agent Overrider
Valence


Version-Release number of selected component (if applicable):
firefox-45.0.2-4.fc24.x86_64

How reproducible:
So far every time after about an hour.