Unless you disabled Add-on updates completely, Firefox will check for add-on updates every time you open Firefox. You shouldn't need to do a manual "check" for updates.

And if the "Get Add-ons" feature has a problem connecting to the Add-ons website, the server might be overloaded with requests. Just try later or in the wee hours of the morning when most other users may be sleeping.

If you still have problems,

It is possible that there is a problem with the file(s) that store the extensions registry.

Delete the extensions.* files (e.g. extensions.json, extensions.sqlite, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry.

• https://support.mozilla.org/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
• https://support.mozilla.org/kb/Unable+to+install+add-ons

If you see disabled or not compatible extensions in "Firefox/Tools > Add-ons > Extensions" then click the Tools button at the left end of the Search bar to check if there is a compatibility update available.

If this hasn't helped then also delete possible addons.json and addons.sqlite files.

How many extensions do you have?

It is possible that the Add-ons server is too busy to process your extension's data.