Internet Explorer (IE)
1) Change font sizes in IE

IE7: From the Page menu, choose the Text size option
IE6 and below: From the View menu, choose the Text size option

2) Change colour and font settings in IE
From the Tools menu, choose Internet Options
Select the Accessibility... button (bottom right)
By checking the desired checkboxes, you can force the browser to ignore the colours, font styles and font sizes

3) Disable images in IE
From the Tools menu, choose Internet Options
Select the Advanced tab (top right)
Scroll down to the Multimedia section
Untick the checkbox labelled Show pictures

Mozilla Firefox
1) Change font sizes in Firefox
From the View menu, choose the Text Size option

2) Change colour and font settings in Firefox
On OSX: With Firefox in focus, select the Firefox menu, followed by Preferences
On other platforms: From the Tools menu, choose Options

Select the Content tab.
To select fonts:
Within the Fonts & Colors section, select the default font and size from the drop-down menus. Clicking the Advanced... will display a new dialog. To enforce your previous font selection, untick the checkbox Allow pages to choose their own fonts, instead of my selections above

To select colours:
Within the Fonts & Colors section, click the Colors... button. The dialog that appears allows you to select a colour palette that best suits you. To enforce your colour selection, untick the checkbox Allow pages to choose their own colours, instead of my selections above

3) Disable images in Firefox
On OSX: With Firefox in focus, select the Firefox menu, followed by Preferences
On other platforms: From the Tools menu, choose Options

Select the Content tab
Untick the checkbox labelled Load images automatically

Further help
If your browser isn't covered here, or you would like more in-depth information, visit My Web my Way, [http://www.bbc.co.uk/accessibility/]an online guide that explains the various accessibility features provided by your web browser.