To access these, just type about:config into your browser address bar. A long list of key:value pairs will display that describe your current configuration.
To fix the per-tab close button issue and revert back to having one global button that closes the active tab, set browser.tabs.closeButtons to 3. To get rid of the tab scrolling, set browser.tabs.tabMinWidth to 0. You can also speed up your browsing by enabling browser pipelining. Set network.http.proxy.pipelining and network.http.pipeline to true. You can also increase network.http.pipelining.maxrequests to speed up web page loading, but remember to be nice to those servers!