adding categories

Using Firefox I cannot add a category unless I go to “tools/options/content/and “uncheck ‘enable java script'”. At this point I can now add categories. However, when writing a post and selecting the newly created category (“test” category for example) and adding a post the post now appears in both the “test” category as well as the “default” category. Also when writing a “post” or “page” I do not see the tabs “Visual or Code”. Rather the page is displayed in code format only. Now if I reenable “java script” in the tools/option menu (see above) in the browser and refresh my post then I see both tabs in the post. Having used WordPress before I had not encountered such strange behaviour.