Bookmarks are not part of history. (Edit: bookmarks are stored in the same file as history, places.sqlite, but clearing history through the menu won't remove the bookmarks.)

Clearing history is unlikely to affect your computer's overall performance.

Be careful not to clear cookies if you prefer to stay logged in to site that remember you using cookies. You can uncheck Cookies in Tools > Clear Recent History.

Thank you very much.

There are other things that need your attention.

Your above posted system details show outdated plugin(s) with known security and stability risks that you should update.

1. Shockwave Flash 9.0 r28
2. Shockwave Flash 8.0 r24
3. Next Generation Java Plug-in 1.6.0_24 for Mozilla browsers

Flash Player uninstall:

• http://www.adobe.com/go/tn_14157 (this will remove the Firefox Flash plugin and the ActiveX control for IE)

You can check if there is a file NPSWF32.DLL in the C:\Program Files\Mozilla Firefox\plugins folder and remove that file if present.

Update the Flash plugin to the latest version.

• http://kb.mozillazine.org/Flash
• http://www.adobe.com/software/flash/about/

Update the Java plugin to the latest version.

• http://kb.mozillazine.org/Java
• http://www.oracle.com/technetwork/java/javase/downloads/index.html (Java Platform: Download JRE)