Mobile View Switcher is a lite addon that let you switch a website to mobile view. Default mobile view is iPhone with iOS 11, but you can change it from the options page.
Simply add a desired user-agent to change the mobile view to your desired one. There are many websites that offer user-agent strings for various devices. Simply copy and paste the desired one in the designated area in options page. Then reload any website to see the mobile view UI (while the addon is active).
Toolbar button serves as ON|OFF switch to enable or disable the addon. Grey color for toolbar icon is when the addon is inactive and yellow color is for the active state. Simply click on the toolbar icon to switch states.
In order to report bugs, please fill the bug report form in the addon's homepage (http://mybrowseraddon.com/mobile-view-switcher.html).