We reference "User Agent Overrider" as "UAO".
You can custom the toolbar button menu item in preference panel.
One line one menu item and user-agent string pair. format as below:
# line starts with `#` is comment
menu_item_label: user_agent_string_for_this_menu_item_label
# ^
# the first colon `:` character as separator
Empty line, comment and other invalid format line will be ignored.
If you remove or rename the checked menuitem's label, the extension will deactivate automatically.
Here is the built-in user-agents file
Here is legacy Microsoft IE list which does not ship in latest version
# IE
Windows / IE 6: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)
Windows / IE 7: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1)
Windows / IE 8: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0)
Windows / IE 9: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)
Windows / IE 10: Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; WOW64; Trident/6.0)
Open about:config, find general.useragent.override item.
This what and the only thing UAO actually modifty Firefox configure.
The value should be the same as your choice in UAO, if not, there is a bug.
Open Firefox DevTools, switch to "Network" tab, visit any url, select a request item on left panel, you will see a "User-Agent" below "Requeset headers" section.
This user-agent Firefox finally send out, maybe not the same your choice in UAO, because other Firefox extension may change it again, is not the UAO bug.
Visit url http://httpbin.org/user-agent
This page will echo what user-agent reach the server, also maybe not the same your choice in UAO, because firewall or proxy in your network will change it again, is not the UAO bug.