So after a bit of searching...

I think that message indicates that Firefox is nearing its self-imposed storage quota for website data (or maybe website and add-on data), which I think is calculated based on total disk space. If storage continues to grow, you'll get a different message about erasing data stored in Firefox itself, so you could just wait for that.

I don't know how to expand the quota to avoid the message. Maybe someone else is more familiar with adjusting the storage quota manager.

I don't think that it is a good idea to extend this storage quota.

See "space-alert-under-5gb-message"

• https://searchfox.org/mozilla-release/source/browser/locales/en-US/browser/preferences/preferences.ftl#line-1351

QuotaManager::StoragePressure

• https://searchfox.org/mozilla-release/source/browser/base/content/browser.js#line-984
• browser.storageManager.pressureNotification.usageThresholdGB = 5

Where do you end up if you click 'Learn more' ?

• https://support.mozilla.org/en-US/kb/storage
• https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox