Refer to the Change the fonts and colors websites use article for changing the fonts web pages use (force pages to respect your preference).

What do you mean in search tab?

Is this some sort of random page or "search" site opening randomly.

You may want to check for malware.

• Troubleshoot Firefox issues caused by malware

If the search tab is a search box on your tab bar with a design similar to a 4-color pinwheel, you may have the "Default Tab" extension. Check for it on the Add-ons page. Either:

• Ctrl+Shift+a
• orange Firefox button (or Tools menu) > Add-ons

In the left column, click Extensions. Then, if you don't know that an extension is definitely useful, disable it.

Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Does that flush the box?

If the search tab is the page that shows up in new tabs (Ctrl+t new tab), there's a hidden setting for that.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste newtab and pause while the list is filtered

(3) Double-click the browser.newtab.url preference and enter your preferred page:

• Page thumbnails (default) => about:newtab
• Blank tab => about:blank
• Built-in Firefox home page => about:home
• Any other page => full URL to the page

Press Ctrl+t to open a new tab and verify that it worked. Fixed?

Two gotchas:

• If Firefox ignores the setting, one of your extensions may be overriding it. You can review, disable, and/or remove extensions on the add-ons page:
  
  orange Firefox button (or Tools menu) > Add-ons > in the left column click Extensions

• If the change works but at the next startup is back to Google, you might have a user.js file in your personal Firefox settings folder (your Firefox profile folder). This article describes how to track down and remove the file: How to fix preferences that won't save.

Any luck?

You can use the NoSquint extension to set default font settings and text color on web pages.

• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/