A phenomenal extension for freelance translators is Grammarly. This is not only a browser extension, but it also comes in the form of a desktop app, or website editor that can be used online.
Grammarly provides a free plan (good enough, in my opinion) and a paid membership for advanced features, which can be okay if you have extra money to throw at it.
The main benefit of using Grammarly is that it's seamlessly integrated into all online writing tools and editors. It can help you improve your writing for your cover letter, your CV, and your translations.
I particularly like that you don't have to do anything too complicated: you install the tool, and it will start suggesting corrections with underlines based on different colors. For example, red flags for 'correctness' suggestions.
This is super useful when I'm writing an email and I want to make sure there are no spelling mistakes or grammar errors due to fast typing distractions.
Also, when using the online editor, Grammarly will give an overall score to your copy (1-100). Furthermore, you have 4 different parameters - correctness, clarity, engagement, and delivery, each representing a particular aspect of your copy.
What you can also do with the online editor is adjust the writing based on your goals of audience, formality, domain (Premium feature), and intent. I don't often bother to use the full online editor as it's clunky when doing work, e.g., an email, although I recognize that it can be useful to draft some better-tailored messages based on goals.
What's more is that Grammarly, when installed as an extension will automatically correct words that are easily misspelled while typing. This is very useful in combination with Gmail smart compose, the feature that allows to leverage machine learning algorithms to write emails faster thanks to accurate suggestions.
It should also work on online CAT tools, such as Smartcat, Crowdin, or SDL Trados Online Editor (tested!), so that's one more tool at your disposal for spell-checking and writing improvement.
I hope you find this post useful. And if you don't know Grammarly, you can test its potential by downloading it from the official website.