Use the Vineflower decompiler instead of FernFlower inside IntelliJ IDEA.
The plugin automatically downloads the latest Vineflower updates on IDE startup (unless you configure it not to). Most Vineflower settings are configurable in the plugin settings.
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Using IDE built-in plugin system:
Settings/Preferences > Plugins > Marketplace > Search for "Vineflower" > Install Plugin
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Manually:
Download the latest release and install it manually using Settings/Preferences > Plugins > ⚙️ > Install plugin from disk...