TL;DR
Mozilla's email masking service Firefox Relay creates aliases that forward to your real inbox. The free tier gives you 5 aliases with a 150KB size limit...
Aspect
Temporary Email Services
Firefox Relay
Account and Setup
You don't need an account. Just open the site to get an address instantly. There is no browser requirement and you don't have to install any extensions. It works on any device with a web browser. This includes shared computers and mobile devices where you wouldn't want to install extensions.
You need a Mozilla account and the Firefox browser or the Relay extension to get started. Setup takes a few minutes. The extension auto-fills aliases on signup forms so you can keep using it easily after the initial setup is done.
Free Tier Limits
You get unlimited addresses and unlimited use. Some services limit how long your inbox stays open but they don't limit the number of addresses you can create. NukeMail gives you 24 hours for each address and there is no cap on the total number of addresses you generate.
The free tier gives you 5 aliases. When you hit that limit, you must delete one or pay $1.99/month for unlimited aliases. Those 5 aliases run out fast if you use them for every new signup. Deleting an alias to make room means you lose access to any service using that address.
Email Size
Most temp email services accept emails up to 10-25MB, which covers virtually all verification emails, receipts and confirmations. NukeMail accepts emails up to 25MB, matching Gmail's maximum outbound limit.
The free tier limits each email to 150KB. This is a very small size. Many HTML emails with images exceed this limit. If an email goes over that size it is silently dropped without any notification to the sender or the recipient. The paid tier raises this limit to 10MB so it is adequate for most purposes.
Trust and Privacy Policy
Quality varies a lot by provider. Some temp email services are packed with ads and have questionable privacy practices. Others like NukeMail are clean and transparent. You should evaluate each one based on their privacy policy and their reputation.
Mozilla is a well-known nonprofit with a solid track record on privacy. Their privacy policy is clear and they have no financial incentive to misuse your data. This is a real advantage that few other services can match. Mozilla is also subject to strong accountability as a public-benefit organization.
Address Permanence
Your email address expires after a set time. It isn't suitable for accounts you want to keep long-term. Then again, that temporary nature is the core feature. It prevents addresses from accumulating in databases where they could be breached or sold.
Aliases stay active forever. You can use them for long-lived accounts and disable them later if spam becomes a problem. This permanence makes Relay a good choice for services you want to use for months or years.
Browser Integration
Works in any browser. Copy-paste the address manually. No special integration needed. This cross-browser compatibility means it works on Chrome, Edge, Safari and any other browser, not just Firefox.
Firefox integration is tight. The extension spots email fields and lets you generate an alias with one click. It's a quick workflow if you use Firefox. If you use Chrome or Safari as your primary browser, you lose that integration benefit.
Reply Support
Receive-only. Cannot reply through the temp address. This limits temp email to scenarios where one-way receipt is sufficient, like verification codes and confirmation links.
The paid tier lets you send replies through your alias. The free tier doesn't support replies at all. Because the paid reply feature works this way, Relay aliases act like separate email accounts for two-way communication.
Verdict
Firefox Relay works well if you already use Firefox and trust Mozilla. The browser integration is convenient and Mozilla has strong privacy credentials. The free tier is restrictive though. You only get 5 aliases and a 150KB size limit so you will hit walls quickly.
Temporary email works better for high-volume and quick disposable use. Do you need to sign up for 20 things today? Temp email handles that without hitting any limit. Do you need an address for 5 minutes to grab a verification code? Temp email is faster than setting up a Relay alias.
If you want persistent aliases from a trustworthy provider and are willing to pay $1.99/month, Firefox Relay Premium is worth considering. For anonymous and disposable use where you don't need an account, temp email is the faster and more flexible option.
Use Firefox Relay for the five or so services you care about most for ongoing privacy and use temp email for everything else. You get the best of both worlds this way. Mozilla's aliases are trustworthy for your key accounts and you have unlimited disposable addresses for throwaway signups.
The 150KB email size limit on Firefox Relay's free tier is a major issue. Many verification emails from banks or e-commerce sites exceed 150KB because of embedded images and HTML formatting. When these are silently dropped you're left wondering why the verification email never arrived. There is no error message or indication that the size limit caused the failure.