Lemmy Instance Assistant It does things like if someone links a post and the link takes you to the post on another instance, it adds a button to show the post on your home instance. You can also right click on a page (say, an article on a news site) or image and choose the option to share it on lemmy, which creates a new post. It also has stuff to help you when you click a link to a community but the community is not federated to your server, or you can go to the list of communities on another instance and it will have links to take you to that community on your home instance. That sort of thing. Basically the beginnings of a RES for lemmy.
I also like Dictionary Anywhere, which lets you double click on a word to get a definition, a bit like the one Google one for Chrome.
There are also various container extensions such as a Facebook or Google one, that isolates those sites to attempt to prevent that activity being associated with your activity on other sites. It can be a little annoying to get used to but I use them. The annoying thing is that when you click say a google site from a search result on duckduckgo, it closes the duckduckgo tab and opens the site in a google container, but then you can’t click back to go back to the search results.
The general container tabs extension is good too. It keeps separate cookies per container. So say if you have 3 different microsoft accounts, you can create different containers. Then you can open a new tab in a specific container and it will remember the account you logged into last time in that specific container, but doesn’t affect other containers or tabs not in a container.