Make sure you have the REQUIRED libraries installed...

glibc 2.17 or higher
GTK+ 3.14 or higher
libstdc++ 4.8.1 or higher
X.Org 1.0 or higher (1.7 or higher is recommended)
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For sound in Firefox...

a) If you are using only alsa for sound, you will need to install apulse to have sound in Firefox. 

b) If you are using pulseaudio, sound should work automatically.
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Download and install Firefox...

1. Open a terminal, and cd into the folder where your download will be saved. For example...

cd Downloads

2. Download Firefox (or Firefox-ESR) 64 or 32 bit version. 

NOTE: To download the 32 bit version, change os=linux64 to os=linux in the command(s) below.

NOTE: If you need Firefox with a language other than English-US, find your language in the list at the bottom of this file, then substitute lang=en-US with your desired language in the command(s) below.  

To download Firefox, enter the following command...

wget "https://download.mozilla.org/?product=firefox-latest&os=linux64&lang=en-US" -O firefox.tar.bz2

To download Firefox-ESR, enter the following command...

wget "https://download.mozilla.org/?product=firefox-esr-latest&os=linux64&lang=en-US" -O firefox-esr.tar.bz2

3. Extract the file...

tar xjf firefox*.tar.bz2

4. Move the extracted firefox folder to /opt...

sudo mv firefox /opt/

5. Create a symlink to the Firefox executable...

sudo ln -s /opt/firefox/firefox /usr/bin/firefox

6. Create a Desktop File...

First, enter the following in the terminal to become root...

su

After becoming root, enter the following command...

cd /usr/local/share/applications

If that directory doesn't exist, create it...

mkdir /usr/local/share/applications

...then use the cd command again.  

Now enter the following command to create a desktop file...

nano firefox.desktop

If you are using alsa for sound, enter the following...

[Desktop Entry]
Version=1.0
Name=Firefox
Comment=Browse the World Wide Web
GenericName=Web Browser
Keywords=Internet;WWW;Browser;Web;Explorer
Exec=apulse firefox %u
Terminal=false
X-MultipleArgs=false
Type=Application
Icon=firefox
Categories=Network;
MimeType=text/html;text/xml;application/xhtml+xml;application/xml;application/rss+xml;application/rdf+xml;image/gif;image/jpeg;image/png;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;x-scheme-handler/chrome;video/webm;application/x-xpinstall;
StartupNotify=true

If you are using pulseaudio for sound, remove apulse from... 

Exec=apulse firefox %u 

so that it looks like this...

Exec=firefox %u

Save the file and close nano with the following 3 actions...

Ctrl + x
y
Enter

Firefox should now be installed and available in your menu.

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For other languages replace 'lang=en-US' with...

   Acholi                     lang=ach
   Afrikaans                  lang=af
   Albanian                   lang=sq
   Arabic                     lang=ar
   Aragonese                  lang=an
   Armenian                   lang=hy-AM
   Assamese                   lang=as
   Asturian                   lang=ast
   Azerbaijani                lang=az
   Basque                     lang=eu
   Belarusian                 lang=be
   Bengali (Bangladesh)       lang=bn-BD
   Bengali (India)            lang=bn-IN
   Bosnian                    lang=bs
   Breton                     lang=br
   Bulgarian                  lang=bg
   Catalan                    lang=ca
   Chinese (Simplified)       lang=zh-CN
   Chinese (Traditional)      lang=zh-TW
   Croatian                   lang=hr
   Czech                      lang=cs
   Danish                     lang=da
   Dutch                      lang=nl
   English (British)          lang=en-GB
   English (South African)    lang=en-ZA
   Esperanto                  lang=eo
   Estonian                   lang=et
   Finnish                    lang=fi
   French                     lang=fr
   Frisian                    lang=fy-NL
   Fulah                      lang=ff
   Gaelic (Scotland)          lang=gd
   Galician                   lang=gl
   German                     lang=de
   Greek                      lang=el
   Gujarati (India)           lang=gu-IN
   Hebrew                     lang=he
   Hindi (India)              lang=hi-IN
   Hungarian                  lang=hu
   Icelandic                  lang=is
   Indonesian                 lang=id
   Irish                      lang=ga-IE
   Italian                    lang=it
   Kannada                    lang=kn
   Kazakh                     lang=kk
   Khmer                      lang=km
   Korean                     lang=ko
   Latvian                    lang=lv
   Ligurian                   lang=lij
   Lithuanian                 lang=lt
   Lower Sorbian              lang=dsb
   Macedonian                 lang=mk
   Maithili                   lang=mai
   Malay                      lang=ms
   Malayalam                  lang=ml
   Marathi                    lang=mr
   Norwegian (Bokmål)        lang=nb-NO
   Norwegian (Nynorsk)        lang=nn-NO
   Oriya                      lang=or
   Persian                    lang=fa
   Polish                     lang=pl
   Portuguese (Brazilian)     lang=pt-BR
   Portuguese (Portugal)      lang=pt-PT
   Punjabi (India)            lang=pa-IN
   Romanian                   lang=ro
   Romansh                    lang=rm
   Russian                    lang=ru
   Serbian                    lang=sr
   Sinhala                    lang=si
   Slovak                     lang=sk
   Slovenian                  lang=sl
   Songhai                    lang=son
   Spanish (Argentina)        lang=es-AR
   Spanish (Chile)            lang=es-CL
   Spanish (Mexico)           lang=es-MX
   Spanish (Spain)            lang=es-ES
   Swedish                    lang=sv-SE
   Tamil                      lang=ta
   Telugu                     lang=te
   Thai                       lang=th
   Turkish                    lang=tr
   Ukrainian                  lang=uk
   Upper Sorbian              lang=hsb
   Uzbek                      lang=uz
   Vietnamese                 lang=vi
   Welsh                      lang=cy
   Xhosa                      lang=xh