Legalities of Extensions that modify websites
<p>I am wondering how it can be legal to make or use extensions that modify the functioning of businesses' websites. Specifically, I am looking at Firefox extensions that modify YouTube's functionality. I began wondering how it can be legal to make or use such products. Do these extensions break into the source-code of such websites? Also, do the websites have to give permissions for such extensions to be created?</p>

<p>In a free country nobody has to justify why their actions are legal. They are unless there is a specific, constitutional, law against them.</p>
<p>Browser extensions (barring the malware ones) are there because the user of the browser finds them useful. The function of a browser is to access information on the web and display it in the manner the user wishes; sometimes content creators think they have a right to control the end user experience, which is not only misguided but can amount to illegal discrimation (e.g. under the ADA).</p>
<p>Now there are two main ways website owners can try to prevent this: firstly, they can claim that the browser extension is making a unlicensed derivative work and thus breaches copyright; and if it breaches a technical control, violates the DMCA also. The other is to add wording in their terms of service to prohibit it.</p>
<p>Violation of the terms of service might be enforceable but only against the end user, and (almost) no company wants to be known for suing its customers. One might make the copyright argument stick but this is part of a long battle between content creators and third party add-on manufacturers, which goes back to at least 1984 when <a href="https://en.wikipedia.org/wiki/Sony_Corp._of_America_v._Universal_City_Studios,_Inc." rel="nofollow noreferrer">Disney sued Sony</a> to get them to stop making VCRs. Or more recently, when <a href="https://cdn.ca9.uscourts.gov/datastore/opinions/2013/07/24/12-57048.pdf" rel="nofollow noreferrer">Fox failed to convince a court</a> that a DVR that skipped ads was illegal; the court holding that a fair use defence was likely to succeed.</p>

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