to fix firefox ubuntu cpu usage 100%

Some of older computer (PIII, PIV 478) with low memory (ex: RAM 256 / 512) using firefox default setting always 100% VPU usage, because plugin container.

A plugin is a piece of software that displays internet content that Firefox is not designed to display. These usually include video, audio, online games and presentations that are made in patented formats. Plugins are created and distributed by the companies that make those patented formats. Some common plugins are Adobe Flash, Quicktime, and Silverlight.

here to fix them.
in address bar type about:config then ENTER.
do a search for ‘dom.ipc‘, and you’ll see something like this:

The values you want to change, are the ones related to the plugin(s) you do not want to run in a separate namespace. In my case, that was the two plugins listed.

Just double-click the key, and change the values from ‘true’ to ‘false’, as shown here:

dom.ipc plugin container firefox

Just restart Firefox, and now your plugins will run in “legacy” mode, the same way they did before plugin-container came alone. The only problem is that you’ll be much more prone to Flash crashes taking out the browser itself, so save often, or use a Session Manager to help restore the tabs you had loaded if/when Firefox crashes.

To install session manager Addons go to here

Session Manager saves and restores the state of all windows - either when you want it or automatically at startup and after crashes. It can also automatically save the state of open windows individually.

learn mor about Session Manager go to here

Session Manager