Side View is an extensions developed by Mozilla that lets users to open any site in a sidebar and easily switch between desktop and mobile view. This feature is somewhat similar to Vivaldi's split view, but more limited as you can have only one other tab opened and sidebar's width is limited. Its development stopped in 2019, but is still available to download.
This extension can be extremely useful. You can browse the internet while monitoring your social media feed or chat. There are extensions that allow users to have Twitter or Discord in the sidebar, but I found only one extensions capable of displaying any website and it was discontinued around the same time as Side View.
While still functional, this extension screams for small fixes, most notably (for me) its pop-up menu doesn't respect system/browser theme and prefers-color-scheme is always sent as "light". Moreover, its usefulness could be (relatively) easily and significantly expanded by increasing sidebar's max width and enabling it to show PDF documents.
Side View automatically completes sidebar with social media like on opera - Mozilla Connect and with PDF functionality accomplishes Split PDF Function - Mozilla Connect and Split view tabs and pdf documents in a single wind... - Mozilla Connect.