I haven't used Slimjet, but a quick google indicates Slimjet is based on the Chromium codebase. Chromium is the open-source version of Google Chrome.
Chrome allows you to disable or enable JavaScript on a per-site basis, using wildcards. No need for an extension, such as NoScript. The settings page is at chrome://settings/content/javascript (copy and paste the link, it should work if you are using a Chrome-derived browser).
Of course, the rub is, showing video almost always requires JavaScript. So, while disabling JavaScript will stop obnoxious ad rolls, it will also likely disable a site's non-ad video content.
One way around that is to write a site-specific extension that restores the playability of content video. But that means you have to know HTML, JavaScript, CSS, the ins and outs of browser extensions, and have the patience to reverse-engineer problem sites and fix them when they change.
One possible solution to ads in general, including video ads, is the Pi-hole ad blocker. It's a DNS server that works normally for most domains but dead-ends known ad-tech domains. You install it on your LAN (home or office) and configure all your machines (laptops, desktops, tablets and phones using WiFi) to use it as their DNS server (this can be done by changing your router's DNS server address). It's light enough to run on a Raspberry Pi, hence the name, but you can run it on any computer you might have lying around.
Pi-hole relies on the fact that ads are almost always served from third-party domains. Typically, a page will contain references to JavaScript from one or more ad-servers. When those scripts run, they orchestrate your ad experience. They download the ad content, ranging from banner ads to pop-overs to auto-playing videos. But if the ad-tech scripts don't download, they never run, and the ads never start. And you don't need to install any ad-blocking extensions or be running a special browser!
I haven't tested it, but it's on my to-do list.
Thanks for all that good info.