Для увеличения боковой панели
1. откройте about: config в Firefox и установите для параметра toolkit.legacyUserProfileCustomizations.stylesheets значение true
2. Меню Firefox -> Справка -> Информация для решения проблем -> Папка профиля -> Открыть папку, создаём папку Chrome
3. В папке Chrome создаём текстовый документ и вставляем в него текст
#sidebar-box {
max-width: none !important;
min-width: 0px !important;
}
Сохраняем и переименовываем в userChrome.css
4. Перезагружаем браузер и радуемся неограниченной регулировкой размера боковой панели.

Seems useful, but having the view limited to 10% of my screen width is just not useful in a lot of cases :(

Works inside the sidebar, I can't use it with Tree Style Tab simultaneously.

Great addon, but it would be awesome if the size of a sidebar could be increased. Currently, it’s too small.

Features I would like to see soon:

1. Add navigation, sorted by importancy:
1.1. Back button - Either own back button in Side View or somehow switch focus from main window to Side View. Working backspace and 3rd mouse key would be idel, after sidebar got focus.
1.2. Possibility to write a web-site address

2. Drag&Drop a bookmark from bookmark bar to Side View

3. Open page from Side View in main window.

4. Make Side View resizable. Now it's only 28% of full screen width. I guess this could be done by UserChrome.css but for some reason it's not loading in my Firefox.

I Love what this does. It is actually a very simple concept - but keeping your calendar or a cloud or mail acc handy on your sidebar shortens my workflows extremely.
I wish they would continue working on it, since I have seen that the last update was in 2018 :/

Can't even resize to 50% of the width. Why is it so hard to have functional side-by-side tabs? Can't even enable it in MS-Edge because I can't open edge://flags and I cba trying to fix it

For any one want to remove the width limitation: https://superuser.com/questions/1276800/customize-the-maximum-width-of-the-firefox-sidebar

nice feature, but why doesn't this add on get any updates

Very useful to the browsing experience, great addition to an already exceptional browser!

Wide monitor support, Please allow side view resize to any size of user choice, many have a wide monitor and max width is too small

but cannot adjust width...wait for next version.