The Web Developer Extension by Chris Pederick has been around since the early days of Firefox and has been a staple in developers’ and technical SEOs’ browsers for decades. It’s not primarily an SEO tool in the way SEOquake or the Ahrefs toolbar are. It’s a browser developer toolkit that happens to be enormously useful for technical SEO work.
The features you’ll reach for most in an SEO context are the ability to disable JavaScript and CSS to see how a page looks to a basic crawler, outline images without alt text, display link details, validate HTML and CSS, and inspect cookie and form data. Being able to quickly toggle off JavaScript and see what a Googlebot-like view of a page looks like is genuinely invaluable when diagnosing crawlability or rendering issues.
I’d say the Web Developer Extension is a complementary tool rather than a replacement for dedicated SEO extensions. You won’t use it to check domain metrics or keyword data, but when you need to understand what’s happening technically on a page, it offers capabilities that most SEO-specific extensions don’t come close to matching. The fact that it’s completely free and has been maintained for over two decades is a testament to how well-built it is. If you do any technical SEO work at all, it earns its place in your extension list.