Announced for enabling in Firefox 48 version multi-process Firefox, also known as Electrolysis or e10S, was finally enabled by default in Firefox 51.
Unfortunately multi-process architecture is currently not supported by AVD and some another add-ons. The new version of AVD with multi-process architecture support is under development from scratch.
Possible solutions for this issue are installing of one of previous AVD versions, downgrade Firefox to the version below 51 with disabled multi-process architecture or disable it manually in the current version.
Electrolysis functionality hosts, renders, or executes web related content in background child processes which communicate with the "parent" Firefox browser via various ipdl protocols. The two major advantages of this model are security and performance. Security improvements are accomplished through security sandboxing, performance improvements are born out of the fact that multiple processes better leverage available client computing power.
The first iteration of multi-process Firefox moves NPAPI plugins, media playback and web content to child processes thus separating them from the browser's core.
Check if Multi-process support is enabled
If you're using Firefox 48 or later, you might be using e10s already. Check about:support and look for a number higher than 0 in the "Multiprocess Windows" entry.
Verifying add-on compatibility
The majority of Firefox add-ons are not tested with e10S multi-process architecture, you can check the list of top add-ons and their compatibility at http://arewee10syet.com
Enable/Disable Electrolysis in Firefox
Mozilla did not added any simple way to switch Electrolysis but you are able to do it by yourself by making a few simple changes in the Firefox configuration.
To disable multi-process Firefox, you need to open Firefox configuration page by typing about:config in browser address bar, confirm that you will be careful, type browser.tabs.remote.autostart in search field and set all found preferences including browser.tabs.remote.autostart2 etc to false by double-clicking.
For enabling Electrolysis just change preferences back to previous value.
Please note that you need to restart the browser to apply changes.
Some configurations, if accessibility is used or add-ons are incompatibility, prevent Electrolysis from being enabled.
When Electrolysis is enabled
In case if Electrolysis is enabled you will notice several firefox.exe processes in your system process manager.
Firefox should run just like before but you may however notice a higher than usual RAM usage. Mozilla confirmed that Firefox with Electrolysis will use about 20% more RAM.