1. Type ‘about:config’ in the address bar.

2. Create or edit the general.useragent.override preference.

3. Set one of these values for appropriate user agent:

Desktop Firefox

"Mozilla/5.0 (X11; Linux x86_64; rv:36.0) Gecko/20100101 Firefox/36.0"

Android (Phone)

"Mozilla/5.0 (Linux; Android 4.4.2; Nexus 5 Build/KOT49H) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.166 Mobile Safari/537.36"

Android (Tablet)

"Mozilla/5.0 (Linux; Android 4.4.2; Nexus 7 Build/KOT49H) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/34.0.1847.114 Safari/537.36"

iPhone

"Mozilla/5.0 (iPhone; CPU iPhone OS 7_1_1 like Mac OS X) AppleWebKit/537.51.2 (KHTML, like Gecko) Version/7.0 Mobile/11D201 Safari/9537.53"

iPad

"Mozilla/5.0 (iPad; CPU OS 8_1_1 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) Version/8.0 Mobile/12B436"

Microsoft IE

"Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)"

Chrome

"Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/37.0.2049.0 Safari/537.36"

Firefox OS

"Mozilla/5.0 (Mobile; rv:26.0) Gecko/26.0 Firefox/26.0"

Lynx

"Lynx/2.8.5rel.1 libwww-FM/2.14 SSL-MM/1.4.1 GNUTLS/1.0.16"

4. To restore default user agent assign the empty string to the preference or rightclick on it and press Reset.