A huge thank you to the person who wrote a review below. I've been using this extension for five years, and in the latest version, either there's an unintentional bug, or the developers intentionally introduced a leak that hadn't existed for 10 years.
This extension is currently causing a memory leak in the browser, even when I'm not opening anything and only on one page. It reaches 60 gigabytes for me, after which everything in the OS starts crashing with errors, from the browser to other programs.

I've been racking my brains for 2-3 weeks of torment, I couldn't figure out what was going on, because I've been using all the extensions for years and everything has been working without a hitch for a long time.
And it turns out that's what it is. The extension, which had been reliable for years, is now leaky and buggy, leaking 40-60 gigs of memory.
As soon as I removed this extension, everything has been fine for several weeks now.

Much more through and intuitive than any other grammar and spell checker I have used.

This is a great tool, however it needs some improvements. The button that is displayed when the user types in the text field, overlaps with the button displayed by another add-on - "DeepL: AI translator and writing assistant". Can this be fixed?

I like that it still allows custom sources.

Really nice, and for me at least, it even seems to be free!

I’ve been using LanguageTool for over 2 years and I really liked it — until about a month ago.

Recently, the extension started causing extreme RAM usage while typing. I have 32 GB of RAM, and during normal use I sit at around 18–20 GB. However, as soon as I start typing in a text field where LanguageTool is active, memory usage quickly spikes to 30–32 GB within seconds, which often leads to Firefox freezing or crashing completely.

This happens multiple times a day. In some cases, just 10–15 seconds of typing is enough to crash the browser. I even had to start writing messages in external apps (like Notepad) and then pasting them, just to avoid crashes — which completely defeats the purpose of using this extension.

What’s strange is that:
- this problem only started about a month ago,
- before that, I had zero crashes for over 2 years,
- I didn’t notice any recent updates that could explain this behaviour.

At this point, the extension has become unusable for me due to its massive resource consumption.

That said, aside from this issue, I really liked LanguageTool and I hope this gets fixed soon.

This is a very useful add-on to Firefox. It's much more than a spelling checker, having a very good grasp of grammar and a reasonable ability to determine which word you meant to type. It's not perfect, but it's an excellent tool, as long as you don't follow it blindly.

Très utile.

El millor corrector de català del món.