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.

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 :/

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

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

Remove the width limit and allow changing the zoom level on the sidebar please.

An extremely useful tool, similar to Vivaldi's "Tab Tilling" function but inferior, because it is extremely limited and the size of the window is exaggeratedly small, still, it fulfills its function and I hope Mozilla will update this add-on again.

Ok