I loved Photon. Proton added a lot of nice touches, but I never understood why would you want to turn the tabs into buttons. Tabs is a great metaphor from the real world, and they take less space. I don't see a single advantage of styling them as buttons. The whole point is that they're visually connected to the currently visible page.
Thankfully however I don't care too much because I can still completely hide the tab-bar with my userChrome.css, and use "Tab Center Reborn" to have my tabs on the side instead.
You're not alone. Here are some steps to restore the old tab style:
Here's what I've been using to get the older design, while also keeping some new niceties.