you may not even have to change to another browser or fork, please have a look at some designs in trickypr.github.io/FirefoxCSS-Store.github.io/select a design and follow the page, and you shall find the instructions (usually just downloading/pasting userChrome/Content.css)
and for scrolling tabs, if your problem is very small tab size, then try changing browser.tabs.tabMinWidth in about:config