Open Mozilla Firefox options by clicking the orange Firefox button and clicking Options.
If you like menus more, press Alt on your keyboard once, open Tools menu and click Options instead.
First, click Content tab above and click Advanced... button to the right of the Enable JavaScript button.
Advanced JavaScript Settings window opens. Make sure you deselect everything except the Disable or replace context menus option. This will keep malicious sites from messing around with open Firefox windows.
Click OK to close the window.
Next, click the Privacy tab. Those very concerned about their privacy and possibility of ad-related tracking on the Internet should tick the Tell web sites that I do not want to be tracked box in Tracking section.
Next, select Use custom settings for history from the Firefox will: box.
Right under Accept third-party cookies there is the Keep until: box. Select I close Firefox from there.
PS. You can also clear the Accept third-party cookies box for better privacy, but keep in mind that a few web sites might not work properly after this.
Next, make sure to select the Clear history when Firefox closes check box and then click Settings... button to the right of it.
Settings for Clearing History window opens. Always select Download History and Active Logins check boxes in History section.
Those very concerned about their privacy can check Cookies in History section, too.
Never check this one if you use two-factor authentication on web sites such as Gmail or Facebook!
In Data section, it is recommended to activate Saved Passwords option. Never use web browser for remembering your login details - any malware can steal these within seconds! Use Password Safe for storing and auto-filling logins instead.
Those not playing online games in Firefox can also put a check mark into Offline Website Data box. Remember that enabling this option can delete your saved games and other settings!
Click OK to close the window.
Open Security tab. Make sure that Warn me when sites try to install add-ons, Block reported attack sites and Block reported web forgeries check boxes are selected - these options will keep your internet browsing safer.
Clear the Remember passwords for sites box. This is very important because malware can easily steal all user names and passwords for sites you have visited (including your bank!). Use Password Safe for remembering passwords instead.
Move on to the Advanced tab and open the Data Choices sub-tab. Those very concerned about their privacy can clear the Enable Telemetry, Enable Firefox Health Report and Enable Crash Reporter check boxes to prevent Firefox from sharing usage data with Mozilla developers.
General users can safely leave the latter two ones ticked.
Next, click the Network sub-tab and make sure that the Tell me when a website asks to store data for offline use box is checked. This one can be left unchecked by those who are hardcore online gamers to prevent confirmation messages for saved games and settings.
The last thing to check is to make sure that Firefox updates itself and its add-ons/extensions automatically. Click Update sub-tab under Advanced tab. The following options are available since Firefox 12.
Make sure that the Automatically install updates (recommended: improved security) option is selected. This makes sure that Firefox stays up-to-date and all security bugs are patched.
Then, ensure there is a check mark in the Use a background service to install updates check box. This enables silent updates to Firefox, the completely automated, non-intrusive and most secure method. If you still want to click Yes or Continue in Windows' User Account Control prompts, clear the check box.
Finally, also enable the Search Engines option in Automatically update section. This is not strictly security-related, but you might like the latest and greatest search services.
That's it for the easy part - click OK button to return to Mozilla Firefox.
Disabling geolocation and enabling Click-To-Play in Firefox
For some strange reason, Mozilla has not included an easy way to turn off geolocation in Firefox. The same applies to Click-To-Play, the feature that prevents drive-by-attacks with hidden malicious iframes from running.
You must take steps that may make you look like a hacker among your friends to prevent sites from tracking your physical location and turning on Click-To-Play. 
First, open a new Tab (keyboard shortcut for this is Ctrl+T). Type about:config into Address Bar and press Enter key.
Firefox will disable a somewhat funny warning about warranty. Ever heard of a warrant for an Internet browser? Yeah, me neither.
Click I'll be careful, I promise.
A long list of cryptic options opens. Type geo. into Search field to display only items related to geolocation.
Double-click the very first item, geo.enabled, so that its Value turns to false. This turns off geolocation services.
If you want to enable Click-To-Play, type plugins into Search field. Double-click plugins.click_to_play so that it turns bold and its Value reads true. To turn the feature off later, double-click the line again.
Now Firefox requires an extra click on the "Click here to activate plugins" button to activate Adobe Flash Player, Java, Silverlight and other plug-ins.
Close the about:config Tab and you're done!
To run your browser even more securely and protect it from zero-day attacks, use the free Microsoft EMET.
