When going to check hundreds of checkboxes on a page, you must be upset if there were no "Select All" option.
But luckily, you can install the Firefox add-on CheckFox and check all those checkboxes in one click, as mentioned before.
The following will show you another similar tool — Check All, which is a free Chrome extension for you to check all checkboxes in one click, too.
After installation, you can see the extension icon "☑" on the Toolbar of your Chrome browser, and clicking on the icon on a page with checkboxes, you can check all of them by clicking on the "Check All" button in the drop menu.
In fact, you can also click on the "Inverse Checks" or "Check All in Element" button to check all the checkboxes.
And to uncheck all the checkboxes, you can click on the "Uncheck All" or "Inverse Checks" button.
By the way, for the extension icon "☑", you can change it to ✔ in red, green, blue or some other color, and you can even upload your own image.
But same as CheckFox, the Check All extension dosn't work for all webpages with checkboxes, for example, it works for Untweeps.com, but not Tweepi.com, even though both 2 websites are available for you to unfollow bulk inactive Twitter users at a time.