Opera Browser Offline
Firefox has uMatrix add-on for the same purpose. The Umatrix sites list is a very handy tool where you can set the rules about what sites you want to block or allow in your browsing. The tool lists only allow and deny sites, not the domain. You can also set the last seen of a website. Either you always block it or allow it to be loaded. So, many such features are not present in Firefox as well. If you are on mobile, you should enable Adblock in your Chrome and Opera browsers.
Opera browser offline
The add-on allows you to set a specific browser and site for opening a file or for viewing a document. For example, you can set that the documents of the website www.gutenberg.org will open in Opera browser by clicking the download link on your browser. You can even set that downloads of google.com will be opened with the website as well.At the same time, you can set that your browser will only open the documents of the given website.
Operas next feature set is to keep you connected to the internet even when you're offline. Every time you open a new tab, your last tab will be closed, which makes sure you stay "informed" of all the tabs you have open online. This gives you a seamless browsing experience, so you have no need to worry about whether you're online or not.
Opera browser's customization options are as varied as they are extensive.You can always check out what others are using as well. Here's a list of popular options and how they compare with Opera's features: