Performance Manager for Firefox by Mihai Lungu

Performance Manager for Firefox helps keep Firefox responsive by intelligently sleeping inactive tabs.

Key features:
• Automatically sleep inactive tabs after a configurable period
• Smart memory modes: Balanced, Aggressive, Maximum Savings and Custom
• Maximum awake-tab limit to reduce resource usage
• Quick actions to free memory by sleeping inactive tabs
• Activity Log showing tabs that were put to sleep, restored or skipped
• Tab Performance Manager for monitoring and manually managing tabs
• Per-site rules with custom sleep times or Always Awake protection
• Protection for active, pinned, audible, loading and private tabs
• No analytics, tracking or external servers — processing happens locally in Firefox

The extension does not directly limit Firefox's total RAM usage. Instead, it reduces resource usage by unloading eligible inactive tab content from memory.