diff --git a/master/buildbot/newsfragments/logviewer.bugfix b/master/buildbot/newsfragments/logviewer.bugfix
new file mode 100644
index 00000000000..6e86ca29fb1
--- /dev/null
+++ b/master/buildbot/newsfragments/logviewer.bugfix
@@ -0,0 +1 @@
+Fixed log viewer selection and copy-paste for Firefox (:bug:`3662`).
diff --git a/www/base/src/app/builders/log/logviewer/logpreview.tpl.jade b/www/base/src/app/builders/log/logviewer/logpreview.tpl.jade
index abcdd771e26..2115f084524 100644
--- a/www/base/src/app/builders/log/logviewer/logpreview.tpl.jade
+++ b/www/base/src/app/builders/log/logviewer/logpreview.tpl.jade
@@ -11,8 +11,7 @@ div.logpreview.panel(ng-class="logpreview.log.name=='err.html' && 'panel-danger'
                 a.btn.btn-default.btn-xs(ng-href="api/v2/logs/{{logpreview.log.logid}}/raw", description="download log")
                   i.fa.fa-download
                   | #{' '} download
-  pre.log(ng-show="logpreview.log.type!='h'")
+  pre.select-content.log(ng-show="logpreview.log.type!='h'")
     div(ng-repeat="line in logpreview.log.lines", style="height:18px")
-        span.linenumber.no-select {{line.number}}
-        span.select.no-wrap(class="{{line.class}}", ng-bind-html="line.content")
+        span.no-wrap(data-linenumber-content="{{line.number}}", class="{{line.class}}", ng-bind-html="line.content")
   div.panel-body(ng-if="logpreview.log.type=='h'", ng-bind-html="logpreview.log.content")
diff --git a/www/base/src/app/builders/log/logviewer/logviewer.tpl.jade b/www/base/src/app/builders/log/logviewer/logviewer.tpl.jade
index b0aa6b0a4a2..0f868102610 100644
--- a/www/base/src/app/builders/log/logviewer/logviewer.tpl.jade
+++ b/www/base/src/app/builders/log/logviewer/logviewer.tpl.jade
@@ -8,8 +8,7 @@
   pre.row.log(ng-show="log.type!='h'", scroll-viewport)
     span(ng-if="log.type=='s' || log.type=='t'")
         div(scroll="line in lines", scroll-position="log.num_lines", load-all="loadAll", is-loading="isLoading", total-size="log.num_lines", style="height:18px")
-            span.linenumber.no-select {{::$index}}
-            span.no-wrap(class="{{::line.class}}") {{::line.content}}
+            span.no-wrap(data-linenumber-content="{{::$index}}", class="{{::line.class}}") {{::line.content}}
   div.panel(ng-if="log.type=='h'", ng-class="log.name=='err.html' && 'panel-danger' || 'panel-default'")
     div.panel-heading
         h4.panel-title {{log.name}}
diff --git a/www/base/src/styles/styles.less b/www/base/src/styles/styles.less
index 02c7c59a6c9..422f0ce22b9 100644
--- a/www/base/src/styles/styles.less
+++ b/www/base/src/styles/styles.less
@@ -82,14 +82,25 @@
   -moz-user-select: none;
   -ms-user-select: none;
   user-select: none;
+  display: inline;
 }
-.select {
-  -webkit-touch-callout: initial;
-  -webkit-user-select: initial;
-  -khtml-user-select: initial;
-  -moz-user-select: initial;
-  -ms-user-select: initial;
-  user-select: initial;
+.select-content {
+  -webkit-touch-callout: text;
+  -webkit-user-select: text;
+  -khtml-user-select: text;
+  -moz-user-select: text;
+  -ms-user-select: text;
+  user-select: text;
+}
+/* https://danoc.me/blog/css-prevent-copy/
+Firefox will insert newlines if the selection go over a no-select span, so we need to implement them with ::before
+*/
+[data-linenumber-content]::before,
+[data-linenumber-content--before]::before,
+[data-linenumber-content--after]::after {
+  content: attr(data-linenumber-content);
+  color: #c7c7c7;
+  padding-right: 10px;
 }
 
 .avatar {