It's easy to install Firefox browser on Debian 8.
Debian by default comes with Iceweasel web browser instead of firefox. Although it is recommended to use Iceweasel you may have a need to install firefox are here is a simple way how to install firefox on Debian Jessie by using Linux mint's debian import repository. 

Firstly edit your /etc/apt/sources.list file and add the following line:
deb http://packages.linuxmint.com debian import

At this point on my sources.list file it looks like this:
# Mint
# Packages: Firefox,
#deb http://packages.linuxmint.com debian import 

Update your package list:

# apt-get update

If there is the unavailble gpg key 3EE67F3D0FF405B2 problem:
# gpg --keyserver pgpkeys.mit.edu --recv-key  3EE67F3D0FF405B2
# gpg -a --export 3EE67F3D0FF405B2 | apt-key add -
# apt-get update

The above command will fetch mint package repository list. As a last step install firefox browser:

# apt-get install firefox

You may want to DISABLE Linux Mint's package repository after Firefox installation  by commenting a newly added line in apt sources.list.
Firefox Web Browser installation on Debian 8 Jessie Linux 

Note:
All be files fetched with APT will be downloaded to /var/cache/apt/archives/ before their installation. In this case I got the file firefox_37.0~linuxmint1+betsy_amd64.deb which seems a Mint file, but works perfectly on Debian 8.

To run the browser:
You will see the icon, but on the command line do:
$ fire (And press TAB key on the keyboard)
Output:
You should see firefox and firefox.real
Test the two options to make sure which is Firefox and which isn't.


Bonus - How to improve Firefox:

It's all about performance.
In the address bar on the browser enter:
0: about:config 

For the values to create:
Right click and choose New > ENTRY and select the value.

Here is a list of the default values:
1-network.prefetch-next = true
2-Create an integer : content.notify.interval = 50000
3-Create boolean : content.notify.ontimer = true
4-Create an integer : nglayout.initialpaint.delay = 0
5-browser.cache.disk.capacity = 358400
6-network.http.pipelining = false
7-network.http.proxy.pipelining = true
8-network.http.pipelining.maxrequests = 32
9-network.http.max-connections-per-serve­r = 256
10-network.http.max-persistent-connectio­ns-per-server = 6
11-network.http.pipelining.ssl = false
12-dom.disable_window_open_feature.menub­ar = false
13-dom.disable_window_move_resize = false
14-dom.disable_window_open_feature.title­bar = false
15-dom.disable_window_open_feature.toolb­ar = false
and last but not least:
network.dns.disableIPv6 = true

Setup the new values as follows:
1-network.prefetch-next = false
2-Create an integer : content.notify.interval = 50000
3-Create boolean : content.notify.ontimer = true
4-Create an integer : nglayout.initialpaint.delay = 0
5-browser.cache.disk.capacity = 131072 (= 128 RAM. ie. 1024KB = 1MB).
6-network.http.pipelining = true
7-network.http.proxy.pipelining = true
8-network.http.pipelining.maxrequests = 30
9-network.http.max-connections-per-serve­r = 30
10-network.http.max-persistent-connectio­ns-per-server = 8
11-network.http.pipelining.ssl = true
12-dom.disable_window_open_feature.menub­ar = true
13-dom.disable_window_move_resize = true
14-dom.disable_window_open_feature.title­bar = true
15-dom.disable_window_open_feature.toolb­ar = true
and last but not least:
network.dns.disableIPv6 = true

More changes I made:
browser.sessionhistory.max_entries = 5
browser.sessionhistory.max_total_viewers = 5
Right click and choose New > ENTRY and select the value (boolean):
config.trim_on_minimize = True, and restart Firefox
browser.cache.disk.capacity = 131072 (1024 KB (1 MB) x 128 MB. This value changed automatically to the default but the status row still says 'user set')
browser.cache.offline.capacity = 131072
browser.cache.disk.smart_size_cached_value = 131072

Check memory:
about:cache?device=memory

Add Ons:
Free Memory 0.95 (Add On)
Memory restart 1.18
Suspend Tab. Suspends background old tabs automatically to save memory usage.
Firebug 2.0.9
Developer
Fasterfox. Makes some configuration change, like in teh section "Bonus - How to improve Firefox:".
Testwise. Software Testing.
TOR button in Firefox. Secure navigation.
Metamask

Sync:
Create a sync account to save your addons and bookmarks, and synchronize your devices.

Monitors:
# top
# ps -aux | grep 'firefox.real'
Graphical system monitor extension looks very cool in the new notifications' bar. Toggle it to ON in the Gnome Tweak Tool.

More:
Default Google (encrypted) as Search Engine, from the PREFERENCES Menu.
Activate Add On Bar

References:
Web: http://www.davidtan.org/tips-reduce-firefox-memory-cache-usage/

To turn on the Video Acceleration go to the menu PREFERENCES., or type "about:preferences#advanced" in the address bar.

To verify that WebGL is enabled:
The web standard in 3D graphics acceleration. Considered as an alternative to Flash, Unity and Silverlight.
To tell if you’re using hardware acceleration in Firefox: 
Enter "about:support" into your address bar. At the bottom of that page is a section labeled “Graphics.” If you’re using hardware acceleration, this section will contain a message similar to “GPU Accelerated Windows: 1/1 Direct3D 10′′.
References:
Web: http://www.doesmybrowsersupportwebgl.com/
File: does my browser support webgl.pdf
Web: http://superuser.com/questions/401005/how-to-enable-the-new-3d-page-inspector-view-in-firefox-11
File: Enable 3D page inspector in Firefox.pdf
Web: http://www.technomantra.com/tilt-3d-website-view/
File: Website in 3D using Firefox Tilt Inspector.pdf
Web: https://blog.mozilla.org/joe/2010/11/10/how-to-tell-if-youre-using-hardware-acceleration/
File: How to tell if you have hardware accel.pdf
Web: http://learningwebgl.com/blog/?p=239
File: webgl examples for testing.pdf
Guides:
Video card
3D Acceleration and VR
