It really depends on which websites you visit. I have to restart Firefox and Chrome multiple times each week due to memory leaks, and that's without any adblocking. Most of it seems to be related to the JavaScript engines. As you probably know, it's not unusual for the JavaScript code on a web page to have leaks, but this should have been cleared up when you refresh or close the tab, right? I see all the time that Firefox have gigabytes of allocations of "nothing", and it also gets really sluggish over time. Restarting the browser and restoring the session reduces the consumption and makes it fast again.