Up top, once logged in, there is a permanent link labeled "Control Panel" - click this. On the next page, scroll all the way down to the "Main Configuration" section and go into the options for "Display preferences, number of shown threads, languages, colors..." by clicking on the "Edit" link to that option's right.
On that page, look for the option "Default display mode" - there are two settings, the default is "Flat Mode" and the other one is "Threaded Mode". Threaded Mode is the option you are looking for.
However, your browser making other programs run choppy may a sign of several potentially problematic issues. One being you may need a system upgrade to more RAM or a Video Card. Another being that you have some Spyware, Malware, or some other nasty programs running in the background. Do some scans with an updated Anti-Virus and Spyware/Malware/Adware detection & removal program like Spybot S&D or Lavasoft's Ad-Aware.
You may also just have too many legit programs running in the background. Things like Adobe, Macromedia, Yahoo Toolbar (and other Toolbars), ect ect (add-on programs automatically installed and set to start up when installing a program they are bundled with). If you have a ton of Icons surrounding the Clock on your screen, then chances are you have way too many programs loading up on start-up. (I only have 5 icons, Norton's, LAN, Sound, Video Card, and Safely Remove Hardware. Having programs start up automatically only lessens your system's performance unless you have a really really high-end system).
Toolbars are an evil. The only one that I know of that is only a "Toolbar" by name and not a piece of Ad-ware and Malware is the UO Toolbar,
http://www.uotoolbar.org/.
Ad-Aware:
http://www.lavasoft.de/
Spybot S&D:
http://www.safer-networking.org/
