I find I'm most productive by postponing heavy browser usage until later in the day. So I start by postponing mindless browsing until then. When trying to avoid distractions, I only open my browser for a specific tasks and close it when it's complete. I don't open my browser if the tasks or inquiry can be postponed until later (for me, innocent sounding inqueries often end up being time-killing hydras - postponing it keeps my priorities in check, and often my interest in the task or inquiry doesn't even survive the day (on the contrary, an unimportant inquiry or tasks can easily suck up a couple hours)). I found logging my browser usage, both time spent running and why I was using it, to be helpful too. I don't do that often though, but when I feel I've wasted too much time surfing I try and crack down.
On top of that, I try and write scripts to avoid opening the browser - it also saves me from having to enter stuff into the log :^) The most frequently used example is a youtube scraper (download the first x videos for query y).
I'm also a heavy Emacs user, so I apply some of the methods towards avoiding endless elisp tinkering, as that can be a timesink too! On the topic of Emacs, I will add one last thing: keeping an agenda in org mode and deliberately clocking in and out of tasks helps me stay focused on what I'm doing - I seem to respect the clocked in time much more.
https://nosurf.org/